Check locally with folks who put on horse shows, see where they rent from. Often the rental companies will sell older equipment, for a lesser price than new. You do have to look at it, make sure pieces are not bent, worn beyond safety, when purchasing from them.
Maybe a search under State Name, Stall Rental or similar titles, could give you a good start. Or calling Show Organizers, the contact names in show listings, to see where they rent their equipment from, will get you started.