Go to Orange in Santo Domingo. Ask your cab driver or hotel reception for the nearest location.
I'm not familiar with the T-Mobile phone set-up. But if you have a SIM card here in the US, and if your phone is locked, see my previous message.
An Orange SIM card costs about RD$350, I believe. You buy the calling cards in various increments. I believe the highest is RD$275, but it has 300 minutes/credits or so. The lowest is either RD$50 or RD$100. Calls are cheap as hell there.
If you speak/read Spanish, check out the Orange website:
www.orange.com.do
I only know this because I just went through this process last month. Hope it helps.