When travelling to the United States, where do you get sim cards?

When Apple released what's new with iphone 4,one of the major new selling features was that the phone would come fully unlocked out of the box. So as a Canadian heading down to the United States I find myself wanting to use my phone. I would normally pay outrageous roaming rates if I used my own sim card. Where do I obtain a micro sim card capable of running my iphone 4. Is the only place to get it AT&T where I will have to sign a contract and pay monthly fees or can they be purchased elsewhere? If so, are they affordable and what kind of minutes are pre loaded onto them?

Thanks