Rogue Music:
www.roguemusic.com
Full market prices, but good stuff.
Downstairs there's "Armens", he has a lot of stuff, but after one deal with him I would not recommend his store to anyone.
Two Lines Music, great collection of analog synths, and electric pianos. Full market prices.
Like someone else mentioned, you should check out Craigslist for stuff in the NYC area. If you have a car, sometimes you'll find a good deal on stuff a little ways outside the city.