An International Intergovernmental Organization is just that; a coalition of foreign governments which work together to promote common ends. That can be humanitarian, political, economic, educational or social good. The biggest and oldest example is that of the United Nations; which was preceded by the League of Nations (after WWI). There are many dozens of these organizations now; several of the largest began within the UN but are now semi-autonomous (such as the World Bank, World Health Organization; and the International Monetary Fund)
Claremont has an extensive collection of documents from Intergovernmental Organizations (IGO's). Most of the documents are classified in LC classification order and shelved in the main library stacks. There are two collections of uncataloged items (pre-1970) IGO documents from a variety of organizations that are located in storage, and the League of Nations documents, that are shelved with the British government douments collection on Mudd One. Most can be located using the online catalog; Blais, and OCLC Worldcat for the others, then one must simply ask someone who might know.