Were can I buy a SIM card for AT&T iPhone 4?

It takes the smaller sims and were I stay there's no AT&t or apple store not to big of a city

Well unfortunately it is an AT&T store where you can get an AT&T SIM. I think there should be an option to have one shipped. Check their website.

You have to call AT&T to ship one to your address or you can unlock your iphone here cheap and use

a different carrier that is locally available to you

http://www.tronicscity.com/at-t-iphone-factory-unlock/