T-Mobile and a number of resellers also offer prepaid cell phone service.
See: http://www.cellguru.net/prepaid_compare.htm

You can buy T-Mobile, AT&T, and some other prepaid cell phone kits and refills at Target or Wal Mart (which are large stores commonly found at suburban malls). Cell phone stores should also have them. Some of them also sell off of their web sites.