sometimes i just read the posts here and the discussing around the topic sometimes becomes frustrating.
in cabarete at the callejon entrance, 50m inside from the mainroad is the crazy german guy its a pizza place from outside when you go in he has about 50 fishtanks in the restaurant and he also sells, but not cheap.
he even costum builts the fishtanks.
in puerto plata at the vet place at the big intersection where all the annoying window washer kids hang out they also some fish, but i highly recommend to quarantine those before you join with your other fish as there quality is poor, i did get some nice angel fish from them.
for some really cool fish you can also go to bravo supermarket in santiago, they are expensive but have some very cool fish i have not seen around here before.
there is another cool place in santiago, passing the plaza international heading out of santiago turning at a helados bon, to the right about 300m down the road, very good with fishtanks and accessoires