There are two things. Tesla's own website has kind of a "This is what we hope to do" kind of map that's pretty vague and not very reliable of if or when it's actually going to happen.
But in the closer near term, and MUCH more solid, is this site:
www.supercharge.info
Whenever people are scouting for building permits all over the place, when they find something, they start a forum thread here, and it gets added to that map with a link to the thread. Red dots are existing, blue dots are permits found, and cones are already in construction. I see on that map there are already Supercharger permits found in Kalispell and Shelby.