Where to get a sim-card with service for my iPhone 4?

I'm travelling to the US and I want to have internet service in addition to being able to make phone calls on my own phone. Can I get that from Verizon or other carriers without having to change my phone?

Look at eBay or Amazon. And you can't. It has to be unlocked.

The phone would have to be unlocked, so it would have to be GSM as well. Verizon is a CDMA carrier, so they wouldn't be able to help in this case.

Also, if you do get a SIM of course you wouldn't have your current number assigned, you'd have to use whatever number comes with that SIM.

Again, the key to making this work is if it is unlocked or not. No unlock, no go.