The surveyors office has many old maps and drawings, some are available digitally. An example is the 1910 map of Missoula County, which can be downloaded here: 1910 Missoula County Map. This 4' x 6' map was donated by the family of Fred F. Knuckel and is currently on display in the reception area of the Missoula County Administration Building at 199 W Pine Street.
Google Earth Roads - This file contains historical roads from the Road Plat Book 1 and GLO Plats. Some townships have not been completed, so do not consider this a complete and comprehensive list.
Road Plat Books - This file contains all of the Missoula County Road Plat Books. There is a Read Me file that describes the road books and their contents. The books contain road information from around 1888 to around 1914.