Try the metal pile in the local dump. My favorite bike, or at least a lot of it came from the dump. For a couple of years I rode a bike that was 100% dump sourced.
Try yard sales. The season is just starting in some parts of the country. Amazing what you might find.
If there is a college nearby, check the dumpsters when this semester ends as some idiot college students often just toss their bikes.
Check out a book on bicycle maintenance from the library and learn to repair an old bike.