Think Tanks, or research institutes, can be great sources for academic research.
Because many conduct research in order to advocate for particular causes, it is important to know if they are partisan, bi-partisan, or non-partisan, and also if they are affiliated with specific corporations or institutions. If their website does not make this explicit, you can try Wikipedia.
The Library at the Harvard Kennedy School has created a Think Tank Search guide that provides a list of think tanks, a custom Google search scoped to search ONLY think tank websites, and many other sources useful for evaluating think tanks and their work.
The Library at the University of Oregon has compiled directory of organizations that advocate for American Policy.