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 1929 Wisconsin Blue Book, on page 816, but the Blue Book excludes the write-in votes.
La Follette founded the Wisconsin Progressive Party in 1934 and switched to that party from Republican prior to his reelection that year.
Source for Popular Vote data: Wisconsin Board of State Canvassers, Certificate of Board of State Canvassers for United States Senator General Election - 1928 (Madison, 1928)