Yes, there is a large supermarket on the highway in Sosua. Playero.
The best place to buy beef is a German butcher near the entrance to Sosua coming from Cabarete. As you come into town, you will pass a large car wash on your left (there will be one on the right earlier, but that isn't it. Next to the car wash is a small snack bar, and next to it is a long, low building that includes a restaurant. There is a small fence in front. Go through the gate and up the couple of steps to the porch. The door to the meat market will be to your right, the one to the restaurant to your left. We buy beef tenderloin there because it is aged. What you buy in the supermarket isn't, so it is tougher.
For fish, talk with restaurant owners an see where they buy it. I know there is a fishermen's collective in Charamicos (the other part of Sosua) where you can buy fresh fish. You won't find it in the supermarket.
There are small grocery stores in Cabarete, but I'm sure their beef, if they sell it, is not equal to that of the German butcher. Meat isn't routinely aged here like in the US. Consequently it is range fed and tough.
You will find when you cook meat, even ground beef, that there is no fat.