I'm not sure about the sparklers, but Homeland Security might not like the idea.
As far as the legality in the DR, about two years ago they had a crack down. The police seized all fireworks for sale by vendors. It was supposedly in response to publicity about kids getting hurt, but it also might have had something to do with the collection of fees and taxes. Who knows the real reason?
Anyway, in answer to your question, I think the availability and "legality" of fireworks will probably depend on who has an investment in their sale and use.
My advice is to not bring in anything that even looks like fireworks, and then just wait and see. If you see them for sale publicly, it will be safe to buy them, but I wouldn't buy any through any private arrangement.