The Reef Shoppe in the Bronx is good but it's in the Bronx, not much further than Gills in Mamaroneck. Gills is good too though.
Where in Mahopac are you and what are you looking for? In terms of coral, I could probably hook you up with some frags, I come up there very often to see my gf.
if you're new to the hobby make sure your tank is fully cycled before putting corals into the tank. if you already have a tank setup you can use some rock from that tank to help seed and cycle the tank. Take your time do not rush into buying coral.