Not sure if you're interested in folding bikes, but 2 shops - BFold on 13th Street and NYCEWheels on 84th Street - are unlike any shops that I've seen anywhere. Both are fairly compact places and crammed with merchandise, and both employ staff with considerable expertise in their niche area. I've bought two bikes from one of them (BFold) and have received great service. NYCEWheels seems to also have a devoted following.