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.
Advocacy Across Borders: NGOs, Anti-Sweatshop Activism, and the Global Garment Industry by The SLU Libraries have many relevant books on NGOs. Here is a sample of what we have to get you started.
The Change Imperative: Creating the Next Generation NGO by 
Protest and Organization in the Alternative Globalization Era: NGOs, Social Movements, and Political Parties by