How to get a sim card for my at&t iphone 4?

I recently just bought an iphone 4 on craigslist and the owner told me it was an at&t. Does it cost a lot to get a micro sim card WITHOUT anything attached to it?(i just want a sim card, not a plan to go with it)
and also, does at&t have a prepaid plan for iphones or is it only for their GoPhones?
-thanks

I'm pretty sure that AT&T won't give you a SIM without service to go with it.

Their prepaid plan is only for GoPhones. There are other providers that have prepaid SIM cards that work on any unlocked phone - but that means you would need to unlock your iPhone first.