marina is right. Check out www.hispaniola.com. Cabarete is the best place to start. This is the hangout for the kite boarders, wind surfers and such. They will hook you up with what you need to know. Once you get to Cabarete beach, look for the flags and head to those shops.
In the winter, Luperon is quite nice, the best waves are right in front of the Disco from the Hotel Luperon Beach Resort... Great whenever ther's a cold front, or a storm in the north Atlantic...