International Law is comprised of (1) treaties, (2) decisions of "international courts" (e.g., the International Court of Justice), (3) the past practice of nations and (4) the writings of international law scholars. As such, International Law is distinct from "Foreign Law," which is the law of other countries.
It can be exceptionally hard to find international law materials. One good guide is Germain's Transnational Law Research. The Georgetown University Law Center posts a useful collection of guides for researching international law. Globalex posts articles on key areas of International Law. Sometimes I have had success searching the catalogs of the Library of Congress, Harvard Law School, Columbia Law School and Georgetown University's John Wolff International and Comparative Law Library. Also check EISIL, which is "an open database of authenticated primary and other materials across the breadth of international law." When all else fails I've had success relying on the kindness of the International Law Librarians at these and other law schools.
Other places to call include a relevant U.S. government office (e.g., the State Department) or government library -- or a relevant government office or library in an involved country. I have also called foreign countries' embassies and missions to the U.N. (Contact information for U.N. offices is posted on the U.N. Web site (select the link for "UNDP Around The World" at the top of the page)). For European materials, call The Delegation of the Commission of the European Communities (371-3804). One last source: The American Society of International Law Library (202-939-6005).
Oxford University Press publishes the Max Planck Encyclopedia of Public International Law, described as "an updated, comprehensive work covering the central and essential topics in international law." Oxford also offers a number of international case law databases called the Oxford Reports on International Law.
The Georgetown Law Library posts a useful collection of international law-related Research Guides and Other Resources.
Lexis and Westlaw, have good databases for U.S. Treaties (INTLAW;USTRTY on Lexis; USTREATIES on Westlaw) and documents selected for publication by ASIL, the American Society of International Law (INTLAW;ASIL on Lexis; ASIL on Westlaw). Also, Westlaw offers an online version of International Legal Materials (ILM) and a database of international law-related articles and treatises (INT-TP).
The WorldII International Library provides a free searchable database of treaties, journal articles, case law and U.N. Resolutions.
For more information: International law is discussed in many of the subject-specific entries, such as " Environmental Law," " Copyrights," etc. Export Controls are discussed in the International Trade entry.