Policy and Think Tank Sources
Think Tank Search (a custom Google search engine that helps to surface the publications of these organizations, which may not appear in book form or be published in journals).
Find Policy: a curated collection of the top 14 U.S. public policy think tanks and their publications.
PolicyFile: a Harvard Library database that allows you to search for publications produced by 350 think tank publications. Its unique feature is that it allows you to zero in by political "leanings" (left, right center, progressive, conservative). You can apply this limit before a search (scroll down below the search boxes to find the filter). You can also apply the limit after a search. Look to the left side of the screen, for filters.