Where can I get a SIM catd for my Iphone 5?

A friend of mine sold me an iPhone 5 but it doesn't have the SIM card. Anyone know where to get it?

A carrier store perhaps?

You would need to get that from whatever mobile carrier you are going to use the phone on. I have not seen anything specific about that on the websites of other carriers, but I believe T-Mobile would give you a free SIM if you sign up for service with them.