Cell phone?

You can buy Claro, Orange or Viva SIM card,

you have to be sure first. In the US there is a difference between the CDMA network and the GSM network. I think T mobile is a GSM network as is AT&T, I think Sprint and Verizon and MetroPCS are CDMA. It matters because the sim card may not work in the phone, especially if it isnt unlocked. Now in the DR they are ALL GSM networks. And I m not sure about the rest but VIVA will unlock your phone if it isnt.
Save yourself the hassle just go to Claro and buy another phone for $20(675pesos). You dont need any fancy features do you? Give it away to someone when you leave if you dont wanna take it back with you.

I just bought an Alcatel phone from Orange for RD$490(about US$13) and it included 100 pesos airtime. It doesn't have any fancy features but if all you want is to make calls and send text messages it works well.