You should have specified that you wanted a SIM card; we all thought you were just talking about buying more airtime; in the US you commonly buy it by buying a code card.
GSM phones that use SIMS are still not really that popular in the US, and I have never seen them sold anywhere but at a phone-service store. Most overseas travelers coming here buy their US SIM's in their home countries.
As far as I know, the only service that offers them in the Orlando area is T-Mobile; they call it EasySpeak. You can get them on Ebay. If you want to buy them in person, you have to go to one of their stores to buy them (not the T-mobile kiosks that you find in other stores; but an actual T-Mobile store, and most of them still won't know what you are talking about.) Last I heard, the minutes are not cheap, though it may have gone down. When I checked it a few months ago, EasySpeak charges were running run between US $.45-.65/minute. The base charge to have the SIM activated was at least $50 per chip, w/ about $30 of that being airtime value. I've heard of some situations where the base charge was a $100 minimum. Also, the minutes do expire whether you use them or not, and pretty quickly, too.
T-Mobile also offers prepaid card refills that are not SIM cards, just added value.