Where to buy a sim card for iphone 4 and can i connect it to a phone company?

I live in San Diego, California. Anyone know where to buy a sim card? I can't pay online so would the at&t or tmobile stores have them? If so could I connect to at&t or T-Mobile? Please answer thanks

Descripation
If you export meat, the requirements depend on the destination market and whether you export under the Animal
Products Act (APA) 1999 or the Food Act 1981. If you are exporting meat products from New Zealand, you need to meet
the following general requirements. You must:

be registered with MPI
meet all the requirements of the destination country you are exporting to
comply with the APA or the Food Act 1981 as appropriate.