I'd imagine that many of the slaves would be from populations conquered, or at least raided, by
Mordor's Southron and
Easterling allies. (Note: I see RL replied while I was typing, but I'll leave mine as is).
It, along with many other things we'd like to know more about, is left vague, for a couple of reasons, one being the sheer length of the work, which, despite
Tolkien's oft-expressed optimistic letters to his publishers, "grew in the telling" until it took fourteen years to reach completion. Even then, he later regretted that it was "too short" -- a regret I believe most on this forum share. He did spend a great deal of time and effort after publication filling in blanks in the story, for which fans should be grateful. He said in a letter that he was aware of the importance of agriculture, economics, and similar elements in a society, but couldn't get to everything.
I'd suggest another reason: some things are left vague, or at least not described in the kind of detail seen in other encyclopedic works -- Moby Dick, say -- deliberately, in order to maintain the contours of the story's structural imagery, which consistently points to its sources in the radical imagery of myth.
For instance, "orc-meat" is meant to be the demonic parody of
lembas, the "sanctified"
food of
Lorien: we could probably assume the latter would be as distasteful and inedible to the
orcs as it was to
Gollum: "Dust and ashes! We don't eat that". Similarly, "orc-draught" parodies miruvor.
Orc meat goes as far in the direction of radical mythical imagery as possible:
Pippin eats the "stale grey bread" given to him, "but not the meat. He was famished but not yet so famished as to eat flesh flung to him by an Orc, the flesh of he dared not guess what creature"; and
Ugluk has already expressed pride in serving "the Hand that gives us man's-flesh to eat". We don't know where this "man's-flesh" comes from, but the important feature is the imagery:, which goes beyond the contrast with lembas, to parody the sacremental meal, in which followers symbolically take the body of the leader into themselves. It appears in Biblical symbology, of course, but in many other traditions as well. The demonic parody of this is
cannibalism; the radical mythical form of "orc-draft" would be human blood. I note in passing that the the painful but effective unguent smeared on
Merry's wound is the demonic counterpart of
athelas.
The same sort of mythic imagery appears in the freeing of the slaves in Mordor: the Saviour descends into the "belly of the Beast" to liberate the captive souls, an image which occurs in countless stories, from the Bible to Little Red Riding Hood, in some forms of which all the wolf's previous victims emerge alive from his opened belly. I hope to explore more of this particular imagery, as it applies to LOTR, at some point in the near future.
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