The Three Forks Chamber of Commerce (406/285-4753, www.threeforksmontana.com [1]) is a good resource for information about local businesses and sights.
The visitors center is housed in a railroad caboose just around the corner from the huge landmark Sacajawea Hotel [2].
Links:
[1] http://www.threeforksmontana.com
[2] http://www.moon.com/destinations/montana/the-missouri-headwaters-and-south-central-montana/three-forks-and-the-madison-river-/accommodation-0