Start with websites and publications where the organization has summarized its work, e.g., Analytical Guide to the Work of the Int’l Law Commission, Yearbook of the United Nations or the Repertoire of the Practice of the Security Council. Also consult books and articles on your topic. As you read, note the document symbols for relevant documents so that you can use them to locate the full text of the documents. Also consult the research guides, websites and databases below for more information, including how to find official documents from various international organizations.
See also, Jim Church, Access to Information and International Government Organization Archives, 47:4 DttP, Documents to the People (2019).