Political Science Research Guide

produced at the Think Tanks and Civil Societies Progam (TTCSP) at the University of Pennsylvania

a comprehensive digital library of public policy research containing over 30,000 documents.

2015 post looking at most influential think tanks.

From the Global Development Research Center

More for nonprofits, but provides financials on organizations operating in the USA

Custom search engine that streamlines results by pulling information from only NGO website
NGO sites chosen based on consultative status with the UN's Economic and Social Council (ECOSOC), University of Minnesota Human Rights Library, Duke University's NGO research guide, and the World Association of Non-Governmental Organizations (WANGO)