NGOs have no governmental affiliation, and can operate on a national or international level. They usually have a clearly defined mission and will work to achieve that mission through both work on the ground and think tank-like operations. As a result, NGOs can be a rich source of research information from qualitative studies, as well as the statistics they collect through their work. Examples of international NGOs: International Committee of the Red Cross and Democracy International.
To find potentially relevant NGOs see Bond's Membership Directory of over 350 NGOs working in international development.
The SLU Libraries have many relevant books on NGOs. Here is a sample of what we have to get you started.