There are a few different places where you can buy prepaid SIM cards at San Francisco airport. One option is to go to a store like T-Mobile or AT&T. Another option is to go to a kiosk in the airport, such as those operated by SIM Card Express or Cellular Abroad.
If you're looking for a prepaid SIM card that offers good rates for calls and data, we would recommend Cellular Abroad. They have a wide selection of prepaid SIM cards that work in countries all around the world, and they offer free shipping on orders over $50. Plus, they have very helpful customer service agents who can assist you with setting up your prepaid SIM card and getting started with using it on your phone.