You're in CA, right? There are some very good organizations that can match you with the right horse. I would also make sure that you board, or train with, someone who has reschooled a lot of thoroughbreds. Someone can be a very good dressage trainer, but my experience has been that you really need someone who understands the OTTB.
The Retired Racehorse Project is a great way to research/learn about OTTBs and to connect with very experienced people:
https://www.retiredracehorseproject.org/