The research & publications of reputable think tanks, NGOs, and policy organisations, is often an important complement to academic literature (books and journal articles), especially for research topics that have policy dimensions relating to health, crime, social policy, education, etc.

Especially useful are the main portals to the output of these organisations: Overton Policy Documents; Harvard Think Tank Search; Policy Commons

For the UK, see also the list of think tanks & research organisations on the Politics subject guide