You've got some good choices: Hellgate in Missoula, and I can't remember which ones we visited in Carbondale (maybe Phoenix?) and Pueblo (Great Divide Ski?).
There's also some other towns with pretty decent merchandise selections and / or mechanics. West Yellowstone, MT, Silverthorne, CO (bike and ski), Hutchinson, KS (slightly off route), and Damascus, VA come to mind. West Yellowstone had the best tire selection for tourers I saw, Hutch the best saddles, covers, and safety gear, and Damascus the best camping gear in the east, although most small towns in the west have a store with good camping selections.
We had stuff mailed to us in smaller town post offices. Only ran into real problems once (postmistress in Lolo, MT didn't forward mail). There's an art to guessing how far you're going to ride in the next 3-5 days, and getting someone to mail what you need today or tomorrow. Small town P.O.s are the way to go, IMHO. There's no question which office it is, and everyone in town can give you directions. If you get there after closing time or on weekends, just write them a note saying "Please forward mail for John Doe to Five Towns Over, KS, 12345," and sign it. Small town postmasters will do it; in bigger (or more pretentious) towns, they'll want you to fill out a form, which they'll mail to Memphis, which mails it back, and you've missed the next drop.