It's unusual to buy just a prepaid SIM in the US. There's not anough saturation in the GSM market. Perhaps you should consider a prepaid phone instead. You can get a Nokia 5100 (ancient, I realize) from Tracfone for about $50 at most WalMarts, and either pay ~$95 for a year of service and a couple hundred minutes, or $20-50 for some 40-200 minutes that expire in 60 days. Going the second route will mean that you're issued an new phone number every time you reactivate after expiration.

For overseas calls buy a prepaid long distance card where you buy the phone. Rates vary, and would be in addition to the cost of the airtime if you use your cellphone.

There are other prepaid carriers, obviously. My wife has a Tracfone because it uses the Verizon network where I live and she uses very few minutes per month.