I haven't needed one for a while now but Tracfone was pretty good. Alternatively, I believe T-Mobile US will sell you a SIM card that you can put in any unlocked European phone that supports GSM 1900.

Edit: Since another SIM is not a solution, take a look at sites like Amazon UK and Expansys for an unlocked phone before you go. Also, perhaps get this thread moved to travel tech. Pre paid data has traditionally been an issue in the US but I understand this is changing so it might be worth getting a smart phone to compliment your iPhone (and save on excessive iPhone data roaming).