You might check with RK Bridal - I'm not sure if they only do alterations for dresses they sell or not, but they did an excellent job with my dress (purchased there).
If not them, I would also suggest talking with my tailor who is very good, but I'm not sure if he does wedding dresses - but he's done all kinds of stuff for me, and he does great work. His name is Oscar, he works at Mid-City Tailoring. Phone is 212-719-2215. He's located at 28 W 44th/25 W. 43rd (one of those mid-block, pass-thru buildings) between 5th & 6th.