UNESCO/SIO Guide to Archives
As it is nominally responsible for the archives of all international organizations, the UNESCO website is the best place to start any research.
In 2009, a joint project between UNESCO and the International Council on Archives, Section of Archivists of International Organizations (ICA/SIO) led to the creation of UNESCO’s ‘Guide to Archives of International Organizations'.
The guide gives information for around 80 organizations, including non-UN bodies. However, it is out of date in some areas, and the UNHP guide should be taken as a more usable first port of call unless stated otherwise.