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.
Harvard University offers a customized feature that enables users to search almost 700 think tanks (see link below). You can also search Google Scholar. However, if you only search Google Scholar you are very likely to miss some good think tank reports and journal articles. I've created a custom Google search that limits your search to only MA think tanks and policy centers.