I used to go crazy looking for this unlock. But there ISN'T one unless you spend 65$ for a Gevey SIM card, but that's a big hassle, because if your phone turns off you have to re-unlock it. In other words you have to keep the battery over 5% at all times..
So last week I was talking with my cousin and he introduced me to "H2O Wireless".
They use AT&T's towers so you can use their service on your iPhone WITHOUT unlocking it. Best part is it's 50$ a month for unlimited talk, text, and 50mb data (for iphone).
The guy told me I couldn't get MMS on the iPhone with this service, but I ended up proving him wrong and got it working thanks to an app in Cydia. If you need help with that, then PM me and I'll show you how.
Anyway, if their is no H2O in your area, then just look for another no contract service provider that uses AT&T's towers. (Search Google).