You can register your sled at almost any power sports retailer in New Hampshire. Each NH snowmobile club will usually list all locations you can register your snowmobile on their website.
List of NH Snowmobile Clubs
You get a huge discount on snowmobile registrations if you are a member of a snowmobile club in NH. I am a member of the Suncook Valley Sno Riders and we have a way to register for a club membership online. Once you get your club member number you can register your sleds with that number to receive the discount.