You can buy it off an npc. I only remember because I bought it before I knew what it was used for specifically because it said quest item in the description. Which NPC exactly, I can't recall, but there are only so many shops, so have at it. :)
I bought green mold but apparantly triss doesn't understand it because she's not removing it at all. She just repeats how she can do it but never does.
edit: Oh it's wolf's aloe. Hmm I hope I can get that somewhere too.