Sorry to say, but you'll not find any hobby shops of note in NYC....there is a place called Jans, I believe on the upper east side (you'll be able to look them up in the phone book)but that's about it. With the limited time you have, try to get to the Intrepid Air & Space museum. The Museum of Natural History is another amazing way to spend a couple of days.
Enjoy your time in NY, it's a fabulous place.
Regards,
Firechief, former NYC resident.