Yeah, while some enemy encounters are unique, there are plenty of other generic bandits that will steadily regenerate over time, one of the big factors that affect this is the Republics Control, the higher control is, the slower bandits respawn as the government has better grip on public order. Control gets raised by some bandit groups being taken out, as well as some random events, so if you consistently hunt bandits all the time you may find control creeping up, slowing the rate bandits respawn.
At really low levels of control you can get events where entire districts are overrun, this can be inconvenient as bandits will be at the edges of the district extorting tolls out of you if you are not able to fight them (although a decently equipped party can deal with them easily enough) but it also provides ample opportunity for a slave hunter like yourself.
Now, for keeping control low. There are a few things you can do, once you are set up a bit more and recruited a certain character you can set up your own newspaper this has several settings dictating what kind of rhetoric your paper spreads, some of these reduce control, otherwise you can find the bandit faction to donate to them, and in doing so lowering control.
If you are after a consistent source of slaves which isn't dependent on control, your best bet is normally the bounties you can get from town hall, anybody in those fights who isn't the bounty target can be captured.
