For instance, my uncle is a retiree who goes down to Florida every winter. They were getting hit with roaming fees, because they just had a Canadian cell phone. They managed to buy a T-Mobile Nokia 1661 at a Target store as soon as they crossed the border. They were lucky and bought it for $10, because it was on sale at the time. When you need more airtime as well, many places like Wal-Mart and grocery stores down there sell airtime cards. It's much more cheaper and practical.

Where can I buy a USA SIM card ?

In addition to 7-11 stores you can also get inexpensive phone/SIM packages at retailers like Wal-Mart and Target. In some cases these cost $20 or $25 so they're cheaper than buying a SIM alone from someone on this forum and you get a spare phone for free. (The phone might be locked to the US carrier however.)

e.g. http://www.walmart.com/catalog.....d=11038135