I would look at the go-phones for short travel and buy one that is cheap. There are rules for unlocking phones and buying at full price does have that benefit in most cases. I would try to skype to the sales folks and discuss your options with them. I would not buy online as you may have issues not having a US address and Credit Card.
For service or support questions including existing order status, call: Customer Support: 1-800-331-0500
Information on iPhone unlocking can be found here by copying this link into your browser: https://www.att.com/deviceunlock/client/en_US/
I would contact customer service and be as detailed as possible to make sure you do have phone capabilities while in the U.S. Make sure and advise the rep that you live out of the country and only visiting. Customer Support 800.331.0500