The primary source is the report of the State Canvassing Board, available at the Wisconsin Historical Society in Madison. The results can also be found in the 1917 Wisconsin Blue Book, on page 287. The Blue Book does not give the Militia Vote by county; those votes were canvassed separately but are listed by county in the Canvassers Report. The county figures shown here are the combined totals of the regular and militia votes.
Source for Popular Vote data: Wisconsin Board of State Canvassers, Certificate of Board of State Canvassers for United States Senator General Election - 1916 (Madison, 1916)