Well you would have to get a new one from your service provider but if you already have a phone from them you can usually swap it's card into the unlocked phone.
Most cell phone carrier stores will sell SIM cards for their networks if you have a cell phone contract on their network. Some will even give you one for free if you contact their customer service.
Sometimes stores like that have ebay or amazon accounts. Not everything sold on those sites is used. You might have to do a little looking around. You might even try froogle to see if anything pops up there.
