The quickest and cheapest would be to go to the track, cash in hand, trailer hooked up, and just talk to people and ask if anyone has any horses for sale.
be VERY, VERY careful with this approach. Many tracks, including the main track here in AB, do not allow people in the back, unless a trainer or owner will vouch for you. Even if you get into the back, most trainers are very busy, and don't want to talk. Its extremely easy to pick up an unsound, crazy horse, and you wont have time to figure that out until you've gotten them home.
If you want cheap, or 'rescue', go to an auction and pick one up. the track rejects often get run through auctions, and end up selling for slaughter.
If you really want to maximize your chances of ending up with a good minded, sound OTTB, go to a rescue or an owner that specializes in ottb's, like my BO. she has a couple of her own, home bred colts and fillies on the track every year, and the trainers she knows up there(friends with) let her know when there are horses that are too slow, don't want to run or need time off, so she ends up with a few good prospects every year that we retrain and rehome. The trainers also have their relationship with their client(my BO) and reputation to uphold, so they give her a good heath, soundness, vice and behavior run down, so she knows what she's getting into.