Myoko Shuttle covers all the major airports, Narita, Haneda, Niigata and Toyama. This is the cheapest shuttle service available and they offer 5 daily shuttles to/from Narita. Their transfer service extends to Madarao, Tangram and Nozawa so if you are planning on staying there this is a great service to use.
Narita/Haneda : ¥10,000 one way
Niigata : ¥6,000 one way
Toyama : ¥7,000 one way
Nagano Snow Shuttle have been transferring people to and from Narita and Haneda for many years and were the first dedicated airport shuttle service in the area. They are an established service and offer up to 4 buses a day which can very convenient depending on when you land.
They also have daily shuttles to and from Hakuba and Nozawa Onsen.
Narita/Haneda : ¥11,900 one way