Howdy Maryanne, welcome to permies!
Here is a website that I like.
http://www.landwatch.com/Montana_land_for_sale
You should see a small map of montana in the upper right corner, this has all of the counties.
Hover your mouse over any county and click and it will sort the properties in that county for you. You can also specify cost and size etc and search.
Hope that helps?