There are 8 US Post Offices in Sanders County serving a total of 11,345 residents equating to 1,418 residents per post office. It's estimated that approximately 18,108 packages are handled across the 8 county post offices annually. Sanders County is a county located in the U.S. state of Montana. As of the 2010 census, the population was 11,413. Its county seat is Thompson Falls. The county was founded in 1905. It has an annual county fair with rodeo at Plains."