Here is how Harvard defines a "think tank" for the search:
For the purposes of this search, think tanks are defined as institutions affiliated with universities, governments, advocacy groups, foundations, non-governmental organizations, and businesses that generate public policy research, analysis, and activity. Inclusion is based upon the relevancy of subject area to HKS coursework and scholarship, the availability of the think tank’s research in full-text on the website, and the think tank’s reputation and influence upon policy making. The list represents a mixture of partisan and non-partisan think tanks.
A list of the US think tanks in the search can be found here. A list of the non-US think tanks can be found here.
The site Find Policy also allows users to search think tank policy, albeit in a slightly different way.