I've seen used CVA single shot percussion pistols sell for under $100, sometimes for less than $50. Some Spanish pistols show up at the local gunshops selling in your price range.
You'll just have to keep your eyes open. It couldn't hurt to mention your hunt to the local dealers. They're more likely to see these guns than you are, and might buy one rather than pass on it if they know there's a guaranteed sale.
Don't forget flea markets and yard/garage sales while you're looking.
Primitive rendezvous and historic reenactments may also reveal a gun that meets your requirements.
A classified ad in the local free paper might also find one or more.
Good luck!