here's another I have used in the past, also free.
Free worldwide routable Garmin maps from OpenStreetMap
it's a little more complicated to use, but I like their maps better.
Basically, you select "Dominican Republic" under the "North America" header in the center of the page, then enter your email in the box on the left, then select "build my map" under the box where you entered your email.
They send you an email when your map is ready, with a link.
Look carefully at the choices: you want the zip file that is for the "micro SD chip" (it's the sixth file on the link I received)
Click on it, and it should download to your computer. unzip it, and copy it to the "Garmin" directory on your Micro SD chip.
THIS IS REAL IMPORTANT
The microchip you insert in your garmin MUST HAVE a directory that's named "garmin" - if you copy the gmasupp file directly to the chip, IT WON'T WORK.
You have to create the garmin directory first, then copy the gmasupp file to that directory.
(I can't believe I'm doing this again)