I wouldn't buy a used mattress because bed bugs and other stuff are just too hard to get rid of.
What's your budget for a bed?
CHeck the phone book for local furniture stores.
www.1800mattres.com (though t hey are convenient not cheap)
A fullsize mattress set (thats mattress and boxspring) in good quality, not great quality - will probably run about 300 bucks... and should include the frame. If you have any kind of back troubles or joint troubles, don't skimp on the mattress. you will pay for it later one