Where do I get a SIM card for an iPhone that I bought off a person?

I just bought this iPhone 4 off a person and I don't have a SIM card for it. Where can I get one?

T-mobile, ATT, look on ebay

Small SIM cards are available at cellular phone stores, computer stores, an Apple store, or even on line. You just need to know the service you want to use and the size you need.

I know the better answer. Believe me! In bangladesh there's not any sim for iphone. We, all bangladeshi just do it.
1. Cut the sim to a smaller size ( you can do it because, the cheapset of a sim is very small. And it only in the middle of copper plet. So if you resize it with a blade, your sim always work 100% OK.
2.put it on iphone sim slot.
3. When you want to use this sim to other phone you just Place this mini sim to the right Place when a real sim's copper plet touch. That is it!

A phone carrier.AT&T, T-Mobile to name a few.