two words... pawn shop.

in a place like El Paso (spent most of my life in west Texas) you can find a lot of bikes that were $300-400 new for $100... maybe less if you talk 'em down. It sounds like you have a basic idea of what a good bike is, based on the fact that you recognize crap when you see it (walmart), so that should be helpful for you.

there's also always a chance of thrift stores having bikes (like goodwill), but where a thrift store might have 1 or 2 bikes, a pawn shop of similar size will have 8-12 bikes.