In addition to treaties, custom, general principles, judicial decisions, and scholarly commentary, your topic will likely require research in a variety of such as:
Travaux Préparatoires: the "legislative history" of treaties
The Yale Law Library has a Directory of Collected Travaux Préparatories. This directory offers the most comprehensive listing of compiled drafting histories. Print materials are listed alphabetically by title. Links are provided to drafting histories that are accessible online.
Search the Rodino Catalog with the keyword (kw:) "Travaux Préparatoires"
UNHCR Legislative History: The United Nations High Commissioner for Refugees, through its RefWorld database, provides the legislative histories of refugee conventions.