For $25 you can order a quad band GSM ATT Gophone (Samsung A777) with SIM that will be there when you arrive that you can charge with credits for unlimited use at $2 / day. Then you can unlock it and have a backup phone when you get home or sell it.
(The only thing is that you might only be able to order it to the same billing address as your credit card. - not sure)
The alternative is to stop in at any ATT or T-mobile store - those are the only nationwide GSM providers in the US, they will usually charge $10 just for the sim card. Unlike in Europe, prepaid SIM cards themselves are not available all over.
Buying the whole prepaid phone in a store is OK and they are widely available everywhere from walmart to target to CVS drugstores. But - the GSM phones they sell are almost never quadband so they are useless out of the US even if you unlock them. And the other brands are CDMA and are also generally useless out of the US.