We are not affiliated with Montana Haunted Attractions in any way, but they're the official go-to statewide attraction directory.
Butte Halloween focuses on locations within a 2-hour drive of our hometown of Butte, America, including Anaconda, Whitehall, Deer Lodge, Helena, Bozeman, Dillon, and points in-between.
We have nothing against Missoula, Great Falls, or Billings, they're just not in our jurisdiction.
These guys have the scoop on any place in the state worth a 2+ hour drive.
Destination Montana has a listing for local Missoula events.
Visit Great Falls has a similar one.
Billings has a Facebook Page for local Halloween events and Billings 365 maintains a Halloween listing as well.
This list is the best I found for the Kalispell area.