You can buy it quite anywhere in RD. But be carefull with the immitation.
The secure place is in Puerto Plata, in the Amber Museum (in which you can observe the moskito which was showned at the beginning of the movie "Jurassic Park"). And inside, you can find any information you want upon the amber. If you are interesting, I recommand you to take a local guide in the museum.
Have a nice trip, Pardo (djpardo@hotmail.com)
NB: may be you can do something for me : when you're in DR, look for where I can have the "Chart" of the dominican music (or may be someone who can e-mail me it each two weeks) ?