Or the Hutt Cartel or the various other crime syndicates, since underworld dealings usually center around spice/slaves/weapons/tech (in no particular order).
Agreed--even if it's a Hutt-controlled world, there's still plenty of possibilities for slaves belonging to Imperials stationed there. Quesh seems like another possibility to me. All the NPCs there seem pretty miserable to me, and I'll bet they'd go to some pretty far lengths for luxuries. (Not that owning a slave is an extreme action in this galaxy. I'm thinking of someone without a lot of money/power purchasing a slave--that person would be less likely to be able to hide/otherwise hang onto the slave when the slave's force sensitivity is discovered.)