Find UN Documents from the UN Document Center:
http://www.un.org/en/documents/
Use the legal databases provided by each organizations.
Organization of American States (OAS): Treaties and Agreements
World Trade Organization (WTO): Legal Texts
North America Free Trade Agreement (NAFTA) Secretariat
United Nations Environmental Programme (UNEP):ECOLEX
Food and Agriculture Organization (FAO):Legal Office
United Nations High Commissioner for Human Rights (UNHCHR)
United Nations High Commissioner for Refugees (UNHCR)
United Nations Division for Ocean Affairs and the Law of the Sea
International Labor Organization (ILO)
World Intellectual Property Organization (WIPO): WIPO Lex
International Law Commission (ILC)