Are you interested in recent or historical union statistics? An excellent source for gathering this type of data is the Union Membership and Coverage Database. The website is an “Internet data resource providing private and public sector labor union membership, coverage, and density estimates compiled from the Current Population Survey.” Detailed information, going back to the 1980’s, is available in either .HTML or Excel format.
The site is updated annually by Barry T. Hirsch (Georgia State University) and David MacPherson (Trinity University). To learn more about the database, I recommend reading “Union Membership and Coverage Database from the Current Population Survey: Note,” Industrial and Labor Relations Review, Vol. 56, No. 2, January 2003, pp. 349-54.