Here are some custom Google search engines.
Please let me know if they're helpful, and send me your finds to add.
delicious is a free academic networking site. Users may tag resources (articles, web sites, etc.) with terms most useful to them, and share their library of bookmarks if they wish.
Comprehensive resource for finding political advocacy groups by the issue they work on. Includes contact info, and whether or not they assign ratings to members of Congress. (Ratings by how often they are cited in the media were done in 2006.) By Kathi Fountain of Washington State U.
Think Tank Search
From Harvard's Kennedy School of Government library; custom Google search + list of hundreds of US and international think tanks
Think tanks are defined by the above as institutions affiliated with universities, governments, advocacy groups, foundations, non-governmental organizations, and businesses that generate public policy research, analysis, and activity.
Tips: Search these selected think tanks by keywords to find research and analysis.
If one seems most relevant to your topic, go to the home page and mine the site for publications, issues, research, and links to other organizations.
Global Go To Think Tank Index (U Pennsylvania)
American Policy Directory (U Oregon)
Think Tank List (WorldPress.org)