To find thrift store or garage sale deals, you either have to get super lucky (doubtful), or you have to spend a lot of time looking. Don't underestimate the time you will end up spending looking for such deals.

Or pay more and you can find decent deals on Craigs List or similar. On Craigs List, the best deals go super fast, so you have to be willing to jump in the car and chase after a deal any time of day, not just convenient times (thats when the competition is looking too). And you probably will need to be willing to drive farther distances, not just in your immediate area, to snag deals. And with the best deals, there is not enough time to do research, so the quicker you can spot a deal, the better your chances are of getting there first.

The more of these methods you are able to do, the better your scores.

And you have to be open to bikes not just your size, selling the ones that don't fit to fund bikes that do fit. 90% of the bikes I buy are not my size.

+1 All of the other ideas above are great too. Post a WTB ad in Craigs List frequently, check recycle centers, church sales, ads in the neighborhood newsletter, etc.