New Vocations is a reputable place that sells OTTBs. There are a ton on facebook groups as well.
I've heard some not so good things about Thoroughbred Sport Horses. People ended up with half starved, injured, or crazy horses since you are not allowed to see the horse before purchasing. Check equine.com equinenow.com and dreamhorse.com. Lots of Facebook groups to look into as well :)
CANTER has a ton. Check online listings.
Go to Thoroughbred Sport Horses on google or Facebook! I bought three horses from them and hey are AMAZING and have great horses. If you end up not liking your horse you can trade for a different one in their program :$
New Vocations is based out of Ohio they have off track thoroughbreds and stb's. They have a December adoption special going on right now! Horses as low as $200, and they are all very nice horses :)