We had good luck working through our local Boxer club and finding a breeder that was affiliated
For NYC here is their page with the puppy referral e-mail address on the bottom left
http://www.nyboxerclub.com/
Here is Upstate
Available Puppies
Here is Western
BCWNY
There are also clubs on LI and for Middlesex ... here is the AKC list of clubs
American Boxer Club: U.S. Boxer Clubs
These breeders are typically involved with the breed, will breed in a responsible manner and have fantastic dogs .... just make sure you know what you are getting yourself into - great dogs but not great for all situations
If the breeder doesn't go out of their way to quiz you, have a proper puppy contract, etc i would be hesitant to work with them