Usually I would recommend ebay for the prapaid sim since they are a whole lot cheaper there. But that will not help you now, so go to a walmart,kmart, target etc. and get a prepaid kit (tmobile or cingular). It will come with an okay phone and some credit. You can also buy topup credit at the store or topup via the phone or online with your credit card (at least with tmobile).
prepaids at walmart.
http://www.walmart.com/catalog/catal...atNavId=542371
Phones: Xiaomi Mi Mix 2, Samsung Galaxy A50, ASUS zenfone 3,
Sim cards: AT&T (Contract), 3 UK, Piranha Mobile