The Union of International Associations describes an NGO as "a legally constituted organization created by private persons or organizations without participation or representation of any government...[and] are not conventional for-profit business." NGOs can be organized at any level from local to international.
A useful overview (2006): What is a Non-Governmental Organization? by Prof. Peter Willetts, City University, London
Some resources for identifying NGOs include
Examples of NGOs include
Other names for think tanks include policy institutes or research institutes. A think tank may have an ideological perspective or may be nonpartisan; it's useful to review the About Us section of the think tank's website and look for other views about the think tank in order to place their work in context and better evaluate the authority and credibility of their work.
Resources for identifying think tanks include
Examples of think tanks include
University libraries in the city or region where you were may be useful.
They may have open access databases, guides, digital collections, or recommended websites on your topic. Consider emailing them for suggested resources.