Found in the Road of Sacrifices.

From the Halfway Fortress bonfire go straight ahead and drop down the ledges on the right side, past some poison beasts and crucified undead and into a door at the end. Turn right here and head down to a Black Knight the coal is on a body just past him.

Allows Heavy, Sharp and Poison infusion.

Found in Farron Keep.

From the first bonfire head left through the swamp until you come to a tower guarded by a Darkwraith, the coal is on a body inside.

Allows Crystal, Blessed and Deep infusion.

Found in Irithyll Dungeon.

In the same large room where you find multiple jailers and Karla the Witch is locked up. On the left side (facing the main exit) is a room full of malformed beasts, the coal is on a body in this room.

Allows Dark, Blood and Hollow infusion.

Found in Anor Londo.

From the bonfire (Anor Londo), head up the stairs and turn left. Enter the doorway through here to find the coal on the giant blacksmiths corpse.

Allows Lightning, Simple and Chaos infusion.

https://www.youtube.com/watch?v=Qj6rv60ChqA
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