This page provides links to think tanks, which are non-profit organizations that provide analysis and research intended to influence public policy.
It is often difficult to discern a think tank's political orientation. I've shared some information about the organizations' ideologies in the annotations on this page. You'll also find a more complete listing on page 542 of M. Ahmad's article "US Think Tanks and the Politics of Expertise: Role, Value and Impact", which appears in volume 79(4) of Political Quarterly.
Google Scholar allows you search for think tank reports and scholarly journal articles at the same time. However, if you only search Google Scholar you are very likely to miss some good think tank reports and journal articles. The key to conducting high-quality, comprehensive research is to use the tools as complimentary resources.
Also, Harvard University offers a customized feature that enables users to search more than 400 think tanks.
Use the search box at the top of the list to search all of these sites simultaneously.