Main cities like Nagua and Moca?
Naw, I reckon you mean Santiago and Santo Domingo, right?
First, you may already know that there is no Dominican "MLS" system to which all local agents are plugged. Some, sign up with the US MLS but that's for a whole different client base.
From what I see, most people still rely on the large news papers for rental listings within the two major cities (SD & STGO). There are reasons for that on both sides of the "deal". Ads are cheap and agents take at least one month's worth of rent as a commission, something either party involved are somewhat unwilling to let go.
Listin Diario
Diario Libre
Actually, they mostly have Santo Domingo ads, so much that some don't even mention the city name as the location. You need to develop a knowledge of general locations (named neighborhoods and developments).
... J-D.