I live in downtown Brooklyn, and Dominican products can be found at nearly every BSS. I frequent the following BSS in the Fulton Mall:
1) Corner of Willoughby St. and Lawrence St.
2) Fulton Street in Fulton Mall (near the Bank of America)
3) Corner of Fulton St. and Flatbush Avenue
These stores are all Asian-owned, but all three of these BSS carry Dominican products. All three of these stores are in pretty close proximity to one another, so if you dont find what you're looking for at one, you can just go up a block or two to check the others. There are plenty more BSS in this Fulton Mall area.... these happen to be the three that i frequent most reguarly.
Incidentally, if you are in the area, my favorite Dominican salon (Sisters Unisex) is on Lawrence and Willoughby. Ive been going here for 6 years, and i love them and how they treat my hair. Their products are GREAT (all natural based, ie, olive oil, garlic, avacado, etc.), they are gentle with detangling and combing, and you are in and out VERY quickly. Wash and set is $15, including a blow-out if you choose this. English is very limited, but that is a PLUS for me, because is means very little "mouth" and "back-talk" when it comes to saying "no" to trimming or whatever.
The receptionist speaks English very well, so there is never a problem communicating your needs.
To get to the downtown Brooklyn area, you can take the following trains:
Q, R, B to Dekalb Avenue
A, C, E to Jay Street
1, 2, 4, 5 to Nevins or Hoyt Street (closer)
Good luck!