craigslist can be your best friend. Search around on your local craigslist until you find a bike that looks like its in pretty good shape. Then research that brand and model, or even post the ad on this forum and ask opinions. You can get some incredible bikes for cheap, but you have to be ambitious and check regularly.
You would be surprised how many 10 and 20 year old road and mountain bikes are out there and in incredible shape.