looked into this in detail on my recent US trip. ebay is one option but another is just to buy a prepaid us mobile (virgin are cheapest but others also offer) from a store (addresses on the network web sites) or over the internet (can be cheaper - in which case deliver to a hotel or other address for your first night of stay). chuck away the phone or sell on ebay if you dont need it. however you need to check whether the network you are buying covers the area you are in (pretty similar for main metro areas but differ considerably outside), and also whether non-local calls are charged at higher rate (every us mobile comes with a local no, unlike say UK)