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 1925 Wisconsin Blue Book, on page 578, but the Blue Book excludes the write-in votes. The Blue Book also has a misprint for Blaine in Oneida County and a misprint for Rosa in Milwaukee County.
Source for Popular Vote data: Wisconsin Board of State Canvassers, Certificate of Board of State Canvassers for United States Senator General Election - 1926 (Madison, 1926)