Customary International Law: Research Guides & Background Information
Customary international law “... consists of rules of law derived from the consistent conduct of States acting out of the belief that the law required them to act that way.” (Shabtai Rosenne, Practice and Methods of International Law 55 (1984)). The elements of customary international law include:
In 1950, the International Law Commission listed as evidence of customary international law: treaties, decisions of national courts and international tribunals, national legislation, diplomatic correspondence, opinions of national legal advisors, and the practice of international organizations (“Report of the International Law Commission to the General Assembly (Part II): Ways and Means of Making the Evidence of Customary International Law More Readily Available,”  2 Y.B. Int’l L. Comm’n 367, ILC Doc. A/1316).
Shabtai Rosenne, Practice and Methods of International Law provides a thorough treatment of the topic and the resources, and is an excellent place to start.
Researching Public International Law. This guide from the Columbia Law Library includes advice on researching custom and state practice as well as general principles of law (click on “Custom and State Practice”). Other useful research guides include Claire Germain, Germain's Transnational Law Research (1991-), Guide to International Legal Research (2004), and Contemporary Practice of Public International Law (Ellen G. Schaffer and Randall J. Snyder, eds., 1997).
Anthea Elizabeth Roberts, "Traditional and Modern Approaches to Customary International Law: A Reconciliation", 95 Am. J. Int'l. L. 757 (2001) combines good analysis with references to other standard sources.
To find more materials about customary international law in library catalogs, use the subject heading customary law international.