I sent an email to Marian Struble at the address Alice linked above. Here is her reply:

You may purchase tickets by mailing a check made out to BFOM and mailing it to Bozeman Public Schools Music Office at 404 W. Main, Bozeman, MT 59715. That is if you are in the US somewhere. We are currently checking on the legality of selling tickets overseas.

So, it is apparently legal for out-of-staters to buy tickets. Out of U.S. is still up in the air.

She didn't mention sending a stamped self-addressed envelope, but I think it would be an appropriate gesture.

Bruce

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NOTE: See this message (click) - the county attorney says people out of state have to buy tickets through a proxy - Alice, for instance.
-Joe Offer-