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This Google Custom Search is a maintained by Shippensburg University librarians. This tool searches a selection of highly ranked think tanks and can currently be limited to foreign policy or research-oriented think tanks.
A very useful guide to the major think tanks, both U.S. and international, from the journal Foreign Policy. The website version of the Think Tank Index is currently unavailable. This is a link to the pubished form of the Index.
This is a useful list of think tanks. Don't go here for authoritative information about these think tanks - it is Wikipedia - but use it to identify think tanks that might be of interest to your research.