I cannot speak with authority regarding anything outside New York State, but here you can find cheap pre-paid cells with the brand name TracFone. These make ideal backup cells, especially for putting in a car emergency bag, etc. Heres why:

These TracFones use the unused airtime of multiple cell carriers. When you make a call, the phone checks to see which carrier wants to take the call, and then TracFone gives a portion of the proceeds proportionate to the length of your call to that particular carrier. The practical upshot is youre not tied to one particular carrier; multiple networks can pick up your call and put it through.