The United Nations' specialized agencies are part of a decentralized system of intergovernmental agencies. (Here's the UN's organizational chart.) Each agency is autonomous and has its own constitution and its own specific organizational structure, budget and documentation. Some agencies operate a depository system for their documentation; most of them publish and/or sell their documentation through commercial vendors or operate their own sales systems.
To find other documents at Yale from UN specialized agencies, search Orbis with the name of the agency as either the author or as a keyword phrase in quotation marks. For recent publications, data, and statistics, the agency's website is a good place to start.