So with a side quest I have, something about making fresh breath or whatever, one of the requirements is 2 pieces of charcoal. Anyone know what youkai drops charcoal?

Akamekabe and Dainurikabe
Variations of Nurikabe.

You won't see them until past Mount Kurama in Kyoto

Charcoal sometimes pops up in Miroku's bargains..

Also.. try using this, a lot of people have already asked about this