The best financing conditions would be available with the Asociacion Cibao de Ahorros y Prestamos at probably about 22 percent per year -- given the present high interest situation. This is revised on an annual basis. You can confirm this calling the bank. They will finance up to 80 percent of the purchase for up to 20 years. You will be obliged by them to insure the property in their name. The idea is that if you die, the mortgage will be paid.
I believe you have to be a citizen or legal resident in the Dominican Republic to qualify for a mortgage. For more information, see the real estate pages of the legal section of dr1.com
Dolores may have new information that I ignore but as far as I remember they (Asoc. Cibao) only finance 60%, as much a 70% and right now my friend is paying 24%. And they offer the best deal that I know of. Again, my information may be old.