they are also dropped by an Orcish Cursemaker in Yuhgott Grotto near the burning circle (enterance at the top of Fort Ghelsba) near the Horlais Peak area
I spent 2 1/2 hours killing cursemakers in Yughott Grotto the other night. Not one pair of black slacks, or even a black tunic. I did get about 4 vials of fiends blood off of the bats though. And about 7 keys, but only ever got one cuz I couldn't find a chest to use the first one, lmao.