The CQ publication Guide to Interest Groups and Lobbying in the United States contains a number of definitions of a think tank and similar groups, including an "independent, non-interest-based, non-profit institute that produces and relies on “expertise and ideas to obtain support and to influence the policymaking process."
For more information, see this article.
Find Publications in Group Sites
The services below not only list think tanks and similar groups, but also provide the capability to search for individual publications (papers, articles, reports, books, etc.) located within these groups' Web sites:
Lists of Groups
The services below list hundreds of think tanks, advocacy group, and similar organizations -- perhaps by subject area -- but do not allow you to search across multiple Web sites for partucular topics or publications.