Where can I find a Boxer breeder in New Hampshire ?

You will have to pay more than that for a well bred puppy, especially if he/she is already fixed ($300-400 surgery). Nextdaypets and websites like this are clearing houses for puppy mills. Don't buy from them unless you want a sick puppy. Go to the AKC website and find their breeder referral. It will give a list of breeders in your area. Not all breeders listed will be reputable. Do your homework to find the good ones. In terms of gender, I'd get a boy. Neutered boys almost always get along and even if they don't it's manageable. Girls who don't get along will fight to the death if allowed. Good luck finding your puppy.