Where do I get a iPhone 4 SIM card?

I bought a 2nd hand iphone 4 and it came with no sim, do I get one from someone or where do I buy them?

You need to go to a mobile provider such as AT&T, T-Mobile, Sprint, or Verizon and purchase a SIM Card.

A carrier, aka AT&T or T-Mobile or whichever GSM carrier you choose, though if the phone is locked to any specific one of these then that pretty much makes the decision for you.

You need to go to your choosen carrier (ex: ATT, TMobile) and get a sim card for your iphone 4.