Demons and Archdevils - where to get?

[Infiltrator]

Well for obvious reasons I can't.. acquire that castle now. But no matter, at least I found demons, in the least obvious place. After I did the quest for the sucubuss, I went just to check back at her new man and there he was, he greeted me, gave me a succubi whip and sold the damn demons.

The funny thing was the quest was over and I wasn't prompted to go there after it, good thing I checked.