If $$ is the deterrant, check out the classified ads. When you go looking at used bikes, just make sure you know the frame size you need and something about the components that may or may not be present.

If you have a bike knowledgable buddy, take him/her with you. Sometimes you can walk into a great used bike because the seller has upgraded.

Local sports papers (here we have MetroSports) ads and sometimes bike shops have bulletin boards... whoo, bad sentance, but you get my drift.