Nothing on the Web beats Gale's Encyclopedia of Associations (available only in print), so if you don't find what you are looking for, call your local library. In the mean time, give these sites a try.

InfoPlease Associations -- a selection from Gale's Encyclopedia of Associations. Includes Web Sites.

Gateway to Associations -- American Society of Association Executives Directory

Associations on the Net -- Internet Public Library collection of over 2000 Association Web Sites