One scholar has defined a "think tank" as 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.'" [source]
Another scholar, writing in the Washington Post, is more skeptical of the work produced by think tanks.
These services can search for publications or other materials located within important think tank sites.
These services list important think tanks, advocacy groups, and similar organizations, either alphabetically or by subject area.