You'll need to find six Masterwork Leather Pieces to complete the Battledress. The items will all be in the Plot Items section of your inventory.
Kill Jovi Merice and take it from his body. Be sure to take his body too for "Vendetta".
As you enter the Arl's estate after coming in through the basement, you'll enter a bedroom. After the entrance room, you'll be in a hallway. Across from the door you walked out of will be a small room with a locked door. Pick it and then pick the locked chest for the next piece.
In the room where you leave the papers. You'll have two chances at this one. The second time is after you visit your hideout. Either way, it's in an ornate chest in the corner.
After returning to the estate, go through the first couple of fights and there will be a locked door to your right. Behind it, are a weapons master and three mabari's. Kill them and go to the armor stand on the far side of the room. You'll find the fourth piece there.
You'll see a couple of cut-scenes, and then you'll be in a dungeon. After you get to where Sketch and then Tug are being held, there will be a crank in each of their rooms. Pull both. After the second one, head out the way you came in and go into the next door on the left. You'll fight a few guards and there will be a locked chest on the far side of the room. Open it for the fifth piece.
When you escape the dungeon, you'll be brought to the Chantry. Once regain control, go to the desk behind you and look in the drawer. The final piece will be there.