The cheapest and most direct way to find one is to go directly to the track, usually in the last week of the meet and let it be known that you're interested in buying.
However, the safest way is to work directly with a reputable rescue; because they will have done some of the evaluation of the horse for you, and some of the "let down" process. I don't have a recommedation for a rescue for you, so I would accept JDI's, above.
I also would be somewhat leery of OTTBs from a private seller when the private seller has had the horse a short period of time - most likely a failed or difficult reclaim.
In general, the younger the horse and the less it's raced, the easier the reclaim. A three year old that's never broken its maiden would be ideal. Also, I would want one that had been gelded previously, not gelded shortly before I bought it or that I would have to geld once I got it home. Not hard and fast rules, but general guidelines.