How to active my iPhone 3gs?

I got a new iPhone 3Gs off of craigslist and it had no sim card so I can't activate it. I have an old LG phone and tried using the SIM card that came with it but it keeps saying PUK Exhausted, insert a valid SIM with no PIN lock to activate phone. What do I do?

Go to what company you want, they will get one for you.