This is pretty easy in the US. Just go into a best buy or something similar and purchase a SIM-Card (no proof of anything needed)

My suggestions for a card are either MetroPCS (40$ unlimited Data, Text, Talk) or h2owireless (40$ unlimited Talk, Text, Data + 1000 Intl. SMS + 5$ intl. Credit - ~2hrs talktime). Their SIM-Cards are working in almost any Mobile Phone. You just have to register this card for the first time online (use free WiFi). And you can refill it with either a card which you can purchase in-store (BestBuy) or by credit card. You don't need proof of anything at any point!

Another way would be to use the huge number of free WiFi's which are offered by a lot of Cafes, Restaurant etc. ...