White Titanite Chunk's can be farmed off the Bone Towers in Tomb of Giants. There's quite a few near the Bonfire next to where you saw Patches. The area where you fought those two dudes for Rhea. They have extremely low HP, anything hitting above 300 should one shot them, though they are really aggressive so best to use shield and 1H.
And you can get a White Titanite Slab in the area before you fight Nito.
Blue Titanite Chunk can be farmed on Blue Crystal Golems in the area between Duke's Archives and Crystal Cave.
Blue Titanite Slab can be dropped by Crystal Butterflies in Crystal cave, though it's really rare, but there is one you can pick up in Duke's Archives.
Red Titanite Chunk is drop'd by Chaos Eaters in the pit of Lost Izalith, no bonfire nearby so use Homeward Bones.
Red Titanite Slab can also be drop'd by Chaos Eaters, and it's really rare. It is said one can be found in Lost Izalith, no idea where though. You can get one by giving Hawk Girl a Pyromancy Flame and she'll give you a Chunk or Slab.