I don't know how to buy an iphone 3g?

ok i'm from colombia, and i don't know how to buy an iphone 3g in the apple store USA (Miami). First in the homepage it appear shop online and the price of the iphone says that it cost from US 199 and an itouch cost 229, so the question is that i want to buy an 3g iphone but without the at&t's plan?. i'm going there in one month. how can i buy it? thanks.