Other than drops from Salamanders and bought from the shady man in tavern (side room behind receptionist, receiver of Haren Brog's parcel quest)... You can also craft it. For some reason though, in the Enhanced Edition, eating fisstech does not yield the recipe, as any other potion or potion like concoction does...

Wiki says you need top quality (white gull or 5 socket heavy alcohol drink) plus (in that order): 1 quebrith, 3xhydragenum and 1 vermillion. You can even use dominant substance for added effect but that's useless, since eating fisstech makes you unconscious for some time (and buff expires soon).