If you're willing to scratch build, you might look through a toy store or teacher supply store.
I needed lots of horses for D&D, for use as riderless horses and for cutting up to make "hobby horse" bases so standing figures could look mounted. At a teacher supply store I found tubes of toy horses for $10, 11-15 horses per tube. Size varied; larger were appropriate for 30/28mm, smaller probably 25mm. The store was LakeShore Learning; they don't have the same set on their website.
Sculpting detail wasn't great, but was okay. The important thing was no harness or saddles -- these were supposed to be corralled or pastured horses on a farm. The material is a vinyl that is a little stretchy, but could be cut with a razor saw and could be ground down. They were pre-painted -- though after cutting them up you'd need to touch up.
I know, at $1 USD per horse or a little less, it's almost as much as buying plastic wargaming figures. I considered a some Wargames Factory boxes. The Celt Chariots have 6 horses for $20, the Celt or Roman Auxiliary have 12 horses for $20. USD
The Warlord games figures look pretty nice, but as there's only one horse per set, if you need a lot of horses, it could get pricier than the above.
Also, if there's a hobby store near you that sells single Clix figures, a few of the lines had mounted figures, and you might get lucky. I found quite a few horses that way as well, though they were again $1-2 each. They had medieval barding etc though.
