I would assume most of the slaves are from local sources: Greenskins (who have evolved to deal with extreme attrition and harsh environments and reproduce asexually - it is almost literally impossible to ever run out of these, unless you send entire armies to hunt them down), Beastmen (almost as hardy, and probably more edible), Skaven (who already breed slaves in industrial quantities for their own use - the question is what the Druchii are selling them in exchange; I would not be surprised if a quite a few of the Druchii's skaven slaves (lack of capitalization is intentional - meaning species, not culture) are Skaven refugees and living in conditions that are at least not worse than in the Under-Empire) and Hung (freshly captured from the Chaos Wastes) - in rapidly descending order.
The average freeborn farmer most likely has goblin slaves, comparably easy to control and cheap to replace (they grow from the ground!) - and while inferior to Dawi Zharr hobgoblins they are good enough for light farm work. For heavy farm work you call in the local beastmaster with their somewhat tame trolls. Orcs might be cultivated (in fields - just dig a pit, throw in some orc parts and regularly give them a bit too little food to keep them lean and mean) as living practive dummies for military training and for low-budget bloodsports (not everyone can afford elven gladiators).
Beastmen are still easy to come by (especially if you breed them with local lifestock) but tainted by Chaos and harder to control. More something for intermediate beastmasters, while trolls can be controlled even by goblins i.e. rookies. They would cover some farm work, food production (Druchii are cultural sadism writ large, and what more to make meat more murdery than eating someone (barely) sapient) and more expensive blood sports (somewhat infamous gors and minotaurs might even attract elven gladiators) - they would be useless for construction, with their instinctive loathing for civilization (which would easily be a instant agression response to right angles - might explain some of the curved shapes and spikes on Druchii architecture) while goblins and snotlings can throw at least something together. I could even see snotlings kept as indoor slaves, somewhere between brownies and pets - and always afraid of the cats, because you just know that the Druchii like cats, both are far too alike not to.
I could easily see the breeding and keeping of beastmen requiring a license due to inherent risk of Chaos corruption - and because they can do quite a bit of damage if they break out.
Skaven slaves (that are not infected with bioweapons, regular plagues and / or hounded by clanrats) are not easy to come by, and many a "lucky" farmer that just caught one was found dead in their bed, gnawed to the bones - along with their entire family. It probably requires agreements with the local warlord. The easiest barter good to the ever-hungry Skaven would be food - trading mouths to feed for feed, at a more efficient ratio (an possibly more pleasant taste) than eating them directly. On the other hand, if you are very lucky you get some that are good with machinery, having inherited talent from warlock engineers that passed by the breeder pens or some hands-on (and if they were unlucky, hands-off) experience with maintaining the local Skaven infrastructure. With Clan Pestilens in Lustria, I only really see skaven slaves in the northern half of Naggaroth - the southern parts of Naggaroth got too paranoid after one too many bioweapons.
Imported slaves and Hung would be a status symbol, very comparable to spices - something only the wealthy, at the very least the upper middle class can afford.
The step down from living slaves - or step up, depending on the attitude towards conspicious consumption - would be furniture made from the skin and bones of slaves. We know that flaying of enemies and wearing their skins as throphies is at least practiced by Mengil's Manflayers, a Druchii mercenary regiment.
And then there are possibly slaves bred from Druchii - or even more precious yet, Asur! - that have fallen into disfavor. Albeit, considering how strong the Cult of Khaine is described as, with execution by sacrifice more or less the default punishment for captial crimes, this is more of a speculation based on IRL human trafficking and the Druchii both being racial supremacists, and revancists against the Asur. Might depend on the specific crime, with violent offenses meaning execution and non-violent offenses (e.g. embezzlement) meaning enslavement.
The Druchii would need humans (and would not say no to Druchii slaves, especially their own political rivals) to do the high-dexterity work their artisans cannot be bothered with, busy as they are with commissions for the local dreadlords, with very literal deadlines. Skaven can do the job, sometimes, but they are used to quick and dirty solutions, and might just sabotage you and steal your entire shop before they return to the Under-Empire to sell your merchandise (and tools, and the flesh of your relatives!) for clanrat status.
Slaves could be as much as 80% of the population in places (freeborn farmers with goblins) with fewer and more expensive slaves as the urbanization goes up.
