I researched the storage areas in Miles City when I moved here last January. There are several nice facilities, all reasonably and competitively priced and most had vacancies at that time. What you may want to do is just pick the one that is most convenient to you location-wise.
Just FYI: I've been very happy with the one I chose, which was Notbohm Storage near WalMart/Red Rock Sports off of S. Haynes. The number is 406-234-4480 or 800-310-4482 - ask for Chad.
P.S. Spelling of Notbohm is now correct. Oh, and welcome back to Miles City!
[This message has been edited by Steve Craddock (edited 9/15/2008).]