My wife and I just traveled to Punta Cana the beginning of this month. I can tell you I was less than impressed with the price/selection I found a most places.
Reputable shops? Meh.
Fun places to experience the cigar making and aging process? For sure! Don Lucas Cigars is a fun tour to see the process and you can try some of thier cigars. They sell their own brand as well as most other common brands and a small selection of Cubans. I can tell you most cost less in the US than in the DR, but if you are willing to pay a little more for the experience go for it!
Since I'm a bit of a newbie, I can't attach a link yet, but just google search it, I would also recommend looking at the Trip Advisor reviews as well.
-S&S