Old prison records are housed at the 
Montana State Historical Society's Library 
and Archives in Helena, Montana.

Montana State Historical Society

You will need to send a query and give a specific name that you want researched. 
They will request that you sign a confidentiality agreement before any copies
of records are sent--and they will charge for this service.

NOTE:

Please send as much information as you can to aid in the search.
You will receive your copies much quicker.
Always send a self-addressed, stamped envelope with your request.