Where can I find a Boxer breeder in New Hampshire ?

You aren't going to find a well bred Boxer in that price range. If you want a well bred purebred from a responsible breeder try doubling (or even tripling) that price and looking at the breed clubs breeder referral page instead of sites used by rescues to place dogs (petfinder) and BYB to dump poorly bred dogs (the other sites), and you might find one. Otherwise go with a breed rescue and expect to get an adult dog.