In a couple of weeks I’ll be making a trip for work from middle Tennessee to Helena Montana.  I was curious if any body knows of any gun stores along the way that deal in mainly older rifles and accessories.   I’m planning on stopping by Cody gun museum.  Here is the route I’ll be taking

If you can change your route a tiny bit, schedule a stop in Sundance, WY and get a personal gunshow at Tony's...  You won't be disappointed.  ssrifles on this forum, PM him and check schedules...

Wyoming Armory in Cody, Shiloh & C Sharps in Big Timber come to mind. I think they are still all in business.

Agree with all of the recommended stops, the Cody museum is excellent, it also includes natural history, Indian and Buffalo Bill sections , so allow yourself plenty of time. While in Cody, check out the Irma Hotel ( Really cool carved bar and buffet), and staged gunfight in the street outside. Call ahead and schedule a tour at Shiloh Sharps, well worth the time. 
Not directly gun related, but in Great Falls is the Charles Russell museum with many of his original paintings, and when I was there a couple of years ago, they had a gun collection downstairs with many vintage single shots.

Are you purposely avoiding the Socialist Republic of Kolorado?

Maybe Nebraska, but moreso in Wyoming and Montana, check the show schedules to see if one or two line up on your route and with your schedule.

I think the Cody Winchester show is the annual meeting of the Winchester association and show. So probably all Winchester, but I don’t know that for sure.

Thanks Tony for the additional information. The first vice-president or the incoming President (Cinnabar) of the Winchester Association, also is the new owner of Wyoming Armory in Cody. Keith and Cameron do excellent gunsmithing work and color casing there and worth the time to visit.