Argonians make sense, as Morrowind borders Black Marsh. How the hell did Dunmer get Khajiit slaves?

Rival clans probably sold them, or they raided by sea.

Sounds reasonable.

Morrowind law dictates that slavery is the ancient right of the dunmer, this law was honored/granted by Tiber Septim back in day.

That being said, House Dres in particular (located on the mainland of Morrowind) is known to go on mass abduction sprees, rounding up many khajiit and argonians from Tamriel. From there a good majority of them are shipped to Tear/Dres which is a hub that sends out the slaves to places like the bitter coast in Vvardenfell.

Well, if the dunmer were to kidnap imperials, the Dres kidnappers would be royally screwed by THE LAW!!!. Bretons, orsimer, altmer, nords, and reguards live too far and will have the law on their side, as kidnapping is still illegal. The khajiit, on the other hand, hardly have anyone protecting them, along with the argonians, so it's easier to get them.

Or at leas this is a logical reason why.

People seem to forget that the Dunmer would enslave anyone, including other Dunmer. Khajiit and Argonians were just the biggest slave population. I recall seeing a few human slaves.

In fact, there is one quest where you have to buy a Dunmer slave to be an Ashkhan's wife.

This is true. But, as I pointed out, it's likely that khajiit and argonians are easier to enslave than other races.