Unlocking needs to be done through the carrier you purchased your phone from. Contact them.
As for providers...
1) The cheapest data you can use is the data you pre-load to your phone before you travel. For maps, use the offline feature of the current GPS app you're using (if available), or switch to an alternative app that provides offline map access. There's no shortage of free and paid offline maps available for iOS.
2) Is this a one-shot deal, or will there be more international travel in the future? If the latter, look into KnowRoaming (which overlays your current SIM card) or Truphone SIM (which replaces your SIM card) - both providers have roaming agreements with both AT&T and T-Mobile. If it's a one-shot deal, it helps to know where you're travelling, because every GSM network in the US has some... holes. AT&T has the significantly better coverage between them and T-Mobile, but AT&T data is more expensive. For AT&T coverage, Airvoice Wireless or H2O Wireless would probably be your best priced options depending on your anticipated usage load, with H2O Wireless SIM cards available at most Best Buy locations.
If you want to learn more about US prepaid MVNO options, check out the guide linked in my sigline.
