There are 35 Miracles in the game, most of which can be found or bought but a number are also Covenant rewards too. To get them all you will also need certain vendors and tomes. The two main vendors are Irina of Carim and Karla the Witch. Be sure to recruit them ASAP as well as finding Yoel to get Yuria for one of the tomes.
NOTE: The only missable miracle is Emit Force which you get from Siegwald's quest so be sure to follow that.
To find Irina you first need to pick up the Morician’s Ashes in the Undead Settlement. They are found in the area that the giant targets with his spears – don’t head into the building straight ahead as you enter, instead go left into the graveyard and follow the path to the end to find them. Give them to the Handmaid at Firelink and buy the Grave Key. Then head into the sewers in Undead Settlement (entrance near the stone bridge that leads to Siegwalds first encounter) and open the door, then follow the path to recruit Irina at the end. Karla is found in Irythrill Dungeon, in the room with a host of jailers, but you need to get the Jailers Key from the Profaned Capital. In the toxic swamp area climb the ladder up the building here and follow the roof to the left and up some stairs, then follow this path ahead to find the key on a corpse. Go back and free her to make her move to Firelink Shrine.
The tomes you need are as follows:
Braille Divine Tome of Carim – Just before the Halfway Keep bonfire in Road of Sacrifices is a bridge. You can drop of the edge to find a ledge below with this item on a corpse nearby. Give to Irina.
Londor Braille Divine Tome – You need to get Yoel to move to Firelink from the Undead Settlement (found just below you when you first arrive there). Then let him release your power five times by dying multiple times, after which he will die and Yuria will arrive. She then sells this item for a mere 50 souls. WARNING: Do not give this tome to Irina or she may be kidnapped by Eygon after you buy a dark spell, save it for Karla who will accept it after first refusing.
Deep Braille Divine Tome – In Cathedral of the Deep you come to a ledge overlooking a room with two giants, go around this area and down the steps on the far side to a room with a chest in front of an altar. The chest is a mimic, kill it and it drops this item. WARNING: Do not give this tome to Irina or she may be kidnapped by Eygon after you buy a dark spell, save it for Karla who will accept it after first refusing.
Braille Divine Tome of Lothric – In Lothric Castle just before the Dragonslayer Armour boss door, go to the left (if facing the boss room) to find a gap in the railing where you can drop down to where one of the dragons was. Drop down here and grab the item from a corpse in the room. Give to Irina.
Heal Aid - Starting spell for Herald, purchased from Shrine Handmaid for 500 souls.
Heal - Starting spell for Cleric, purchased from Irina of Carim for 1000 souls.
Med Heal - Purchased from Irina of Carim for 3500 souls (Requires Braille Divine Tome of Carim).
Great Heal – when you enter the watery area in Irithyll of Boreal Valley (after the dark room full of invisible enemies) head to the right side of the area past the spider ladies and find this on a grub corpse.
Soothing Sunlight - Acquired by Soul of the Dancer Transposition.
Replenishment - Purchased from Irina of Carim for 1000 souls.
Bountiful Light - Purchased from Irina of Carim for 5000 souls (Requires Braille Divine Tome of Lothric).
Bountiful Sunlight - Acquired by Soul of Rosaria Transposition. To get this you need to follow the storyline of Leonhard. He is found leaning by the highest throne in Firelink when you first enter, speak to him to get some items. Then after you get a Pale Tongue (can find one near the fire demon in Undead Settlement) and beat the Rotting Greatwood, speak to him again to get a key. Head to the second bonfire in the High Wall of Lothric and head down the stairs and open the locked door. Go down the lift and kill the Darkwraith. Speak to him again back at Firelink. Then find Rosaria at the Cathedral of the Deep and join her Covenant – offer at least one Pale Tongue to her, then speak to Leonhard again by the door to her chamber. You then need to defeat Yhorm and light the Anor Londo bonfire. At this point he will kill Rosaria, so head to her room and grab the Black Orb. Then go to the room above the Aldritch boss fight in Anor Londo and use the Orb. Kill Leonhard to get the soul. NOTE: If you choose not to join the Covenant early on, this can all be done after killing the last boss.
Caressing Tears - Purchased from Irina of Carim for 1500 souls.
Tears of Denial - Purchased from Irina of Carim for 10,000 souls (Requires Braille Divine Tome of Carim).
Homeward - Purchased from Irina of Carim for 3000 souls.
Seek Guidance - In Cathedral of the Deep you come to a ledge overlooking a room with two giants, go around this area and down the steps on the far side, then head left down the next stairs to a room with some thralls and looking down on a Cathedral Knight, it’s on a corpse here.
Sacred Oath - Warriors of Sunlight Rank 1 reward. Find the Covenant in the locked room you can drop into in Undead Settlement (past the bonfire surrounded by workers). Then you need to use the symbol and the White Soapstone to help other players, you get one medal each time you succeed and need 10 to get this spell. You can also farm medals as a rare drop from Lothric Knights. NOTE: You can only offer the medals at the Sunlight Altar found by following a ladder up and around, to the left side of the building just before the Dragonslayer Armour boss in Lothric Castle.
Force - Starting spell for Cleric, purchased from Irina of Carim for 1000 souls (Requires Braille Divine Tome of Carim).
Emit Force (MISSABLE) – You need to follow Siegwalds quest to get this. Speak to him at the lift just after the Undead Settlement. Then head up the lift and then back down to find him on a ledge halfway up. Speak to him and help him beat the fire demon then speak to him again. Then at the Cathedral of the Deep, when you open the main doors (that lead back to the graveyard you came through) head back to the Cleansing Chapel bonfire and then go to the well just outside the doors to speak to Siegwald. Go back to Firelink and head up the locked tower. When you come back down Patches will lock you in. Speak to him then, drop down the ledges nearby to escape. Head back into Firelink and up the stairs on the left to find Patches at the top past the arches. But Siegwalds armour from him. Head back to the well and give it to Siegwald. Then as you head through Irithyll of the Boreal Valley you come to a large water area, head into the sewer on the left side and up the stairs opposite. You’ll find Siegwald in a kitchen, speak to him for this spell.
Wrath of the Gods – In the toxic swamp area of the Profaned Capital, head up the ladder on the building here and then left around the roof and up the stairs. On the roof to your right as you come up is an NPC. Next to him is a small tower part of the roof, it has a gap in it so you can drop to a ledge in the room below and grab this.
Lightning Spear – Go up the elevator near the Old Wolf in Farron’s Keep. Head up the stairs here (watch out for the demon) and go right to the closed gate. To the right of it is a path you can drop to and follow to get behind the gate, the spell is on a corpse here.
Great Lightning Spear - Warriors of Sunlight Rank 2 reward. Find the Covenant in the locked room you can drop into in Undead Settlement (past the bonfire surrounded by workers). Then you need to use the symbol and the White Soapstone to help other players, you get one medal each time you succeed and need 30 to get this spell. You can also farm medals as a rare drop from Lothric Knights. NOTE: You can only offer the medals at the Sunlight Altar found by following a ladder up and around, to the left side of the building just before the Dragonslayer Armour boss in Lothric Castle.
Sunlight Spear - Acquired by Soul of the Lords Transposition.
Lightning Stake - In the Smouldering Lake is a Giant Worm just before the boss door. You can actually keep your distance and just pelt him with arrows while he doesn’t move, then he will drop this when he dies.
Lightning Storm - Acquired by Soul of the Nameless King Transposition.
Divine Pillars of Light – Head onto the top most roof of the Grand Archives and you’ll come to a small building where three Gold Winged Knights drop down to attack you. Drop down the hole in this building, then drop onto a cage and finally drop into the cage to grab this.
Blessed Weapon - Sold by Irina of Carim for 8000 souls (Requires Braille Divine Tome of Lothric).
Lightning Blade - In Irithyll Dungeon you come to a room with a giant and lots of rats, from the upper ledge go down the ladder and across to a sewer opening with more rats. Take the next right to a room with two giant rats, go into the next room and into the door immediately on your left. Head up here to make your way to the Dragon statue, then go past it to a lift and hop off halfway up to find this.
Darkmoon Blade - Blades of the Darkmoon Covenant Rank 2 reward. To join the Covenant you need to speak to Sirris when she first arrives at Firelink (after you speak to Anri and Horace in Road of Sacrifices) then speak to her again after you give the Dreamchaser’s Ashes (found in the Tower of the Old Wolf in Farron Keep) to the Handmaid to get a gesture. Then when you get to Anor Londo, after revolving the platform up, head back down the spiral stairs and across the invisible bridge to a tower and speak to Yorshka to join. You need 30 Proof of a Concord Kept to get this (one is found by the Church of Yorshka bonfire and one in Anor Londo on a corpse) the rest can be acquired by killing invaders or farming the Silver Knights in and around Anor Londo.
Magic Barrier - Sold by Irina of Carim for 5000 souls (Requires Braille Divine Tome of Lothric).
Great Magic Barrier – At the top of Archdragon Peak head past the bell (that leads to the Nameless King fight) and past the bonfire, in the next area head up the stairs on the right and up a ladder to find Havel by a dead dragon. Look on the left side of this area for a gap in the wall and drop down to find this item on a corpse.
Dark Blade - Purchased from Irina of Carim or Karla for 10,000 souls (Requires Londor Braille Divine Tome).
Vow of Silence - Purchased from Irina of Carim or Karla for 15,000 souls (Requires Londor Braille Divine Tome).
Dead Again - Purchased from Irina of Carim or Karla for 5000 souls (Requires Londor Braille Divine Tome).
Atonement – from the Farron Keep Perimeter bonfire, follow the path ahead past three resting Ghru until you reach a hole in the wall on your right. Drop down and find this on a corpse.
Deep Protection - Purchased from Irina of Carim or Karla for 4000 souls.
Gnaw - Purchased from Irina of Carim or Karla for 2000 souls.
Dorhy's Gnawing – In Irithryll of the Boreal Valley head up the stairs to an area with two hags, then left past more hags and a flame user (with another flame caster up on the right). Just past them will be a fenced area on the left side. Smack the right hand fence to open a path down. You’ll be above an Evangelist enemy. Kill them and they’ll drop this spell.
Lifehunt Scythe – Acquired from Soul of Aldrich Transposition.