‘The term “international organization” refers to an organization established by a treaty or other instrument governed by international law and possessing its own international legal personality. International organizations may include as members, in addition to States, other entities’ (Art. 2 lit. a DARIO, Yearbook of the International Law Commission  vol. II, Part 2).
Internat 680 is the section dealing specifically with international organisations, but searches of SOLO may well uncover useful titles in other sections.
The specific subject search for SOLO is (annoyingly) International agencies
In MEPIL, start with article "International Organizations or Institutions, General Aspects" - and work out from there via the links etc
See further the guide written by our colleague in the Bodleian Official Papers