Keyser, is your iPhone unlocked/able to work on the AT&T network in the US?

For local sim cards, I recommend using a prepaid plan with an AT&T MVNO:

H2O Wireless
Red Pocket
Pure Talk
Jolt Mobile

There are others AT&T MVNOs, but that list should be a good springboard. You can purchase sim cards for close to nothing on eBay and load them instead of purchasing a new one from the provider.

You can also try T-Mobile or T-Mobile MVNOs, but if you plan to use data frequently, you are probably better off with the AT&T network (faster on average). One caveat to using MVNOs... you probably won't have visual voicemail, if you care about that.