There is a pet shop in Sosua on the street between Rockys and The adjuntamiento 1 block off Pedro Clisante. They have a bunch of pet supplies, goldfish and turtles. He has dogs and cats, but they are always sad looking, so I wouldn't go there for animals.
Smells bad in the store on most days, so quickly pick yourself a fish and dicker on the price. Tried to charge me 500 pesos for a small fancy goldfish, but you can get the price down to more than half of that. Every week he seems to get a new batch of fish so if you don't see what you want, wait a week. You can buy a tank and filter there, but check for all the parts. He scavenges the sets to use for himself, especially the charcoal and filter floss.
It's a unique experience! I have had my Moby Dick for over a year! Good luck with shopping!
