Closer than many of the suggestions would be Condon's Lumber Supply in White Plains. They stock a variety of marine woods including mahoghany, teak, ipé, okoume & Bruynzeel marine ply as well teak & holly ply for cabin soles. 3/4" seems thick - and heavy - for cabin sole usage. That's what you'd use for code compliance in a house, where there might be pianos. Most cabin soles I've seen use 1/2", with reinforcing brackets on the underside if they're needed for a big span. If you really do require 3/4", you might be able to save by gluing 1/2" teak & holly onto 1/4" marine ply. Or maybe not. They may not want to sell you a half sheet each of two different thicknesses, since they're not likely to have much market for the remaining halves. I've also had good results with Connecticut Plywood, in Milford (right near North Sails), but that would be a bit more of a trip for you.