I live in Brooklyn, but I get all my alterations done at this shop in Manhattan in the Delancey Street area. It's called the Tailor Shop, and it's located on Orchard and Ludlow Street. Really good work, and they work pretty fast.
Prices are reasonable as well. I got a blazer taken in at the waiste, and two jacket sleeves altered. I paid 80, got it back the next day.
It may be a far ways down on the 95th street exit on the R train, but 99th street cleaners for me. The two brothers there live and breath military uniforms and the only people in NYC that I trusted with my USMC uniforms and now with my suits, dry cleaning, and alterations/tailoring (been going there for at least 10 years.) 99th Street and 4th Avenue.