Search "unlocked cell phones" on amazon. They have a fairly large selection of phones that are unlocked to work on any GSM carrier (including SunCom). Be sure to get an american phone, or a phone specifically labeled "quad band", and just plug in the SIM and go!
My son took his sont ericsson w300i swimming and killed it. We are 3 months away from contract end with suncom (who we will not be resigning with) and need an inexpensive option for 3 months. I heard I could buy a pre-pay phone and switch sim cards, but then was told that wouldn't work unless it was a suncom phone. I could get one on e-bay, but he is 14 and impatient. Any advice or recommendations?