Researchers must take care when using secondary sources as evidence of a rule of customary law, taking note of the prominence of the author, the quality of the article, and the prominence of the work.
A few prominent sources include:
Legal researchers in many foreign nations publish unofficial yearbooks that detail the practices and understandings of their states in the realm of international law. The Law Library carries many of these yearbooks, and any number of others can be obtained through inter-library loan. Some yearbooks are also available online through the library's subscription electronic databases.
Here is a sample of the yearbooks that are available:
Australian Year Book of International Law by the Australian National University: KZ21.A98 and on HeinOnline
British Year Book of International Law by the Royal Institute of International Affairs: KZ21.B75 and on HeinOnline
Chinese (Taiwan) Yearbook of International Law and Affairs by the Chinese Society of International Law: Available on Westlaw
Finnish Yearbook of International Law, Jan Klabbers, Editor-In-Chief: electronic resource
Singapore Year Book of International Law by the National University of Singapore: Available on Westlaw
South African Yearbook of International Law by the VerLoren van Themaat Centre for International Law: KZ21.S68
Another great resource for information on foreign state practice are the websites for the foreign affairs offices of the nations you are researching. Some countries' foreign affairs offices are available online, and some are even making them available with English translations.
Here are a few examples:
Australia - Department of Foreign Affaris and Trade: www.dfat.gov.au
China - Ministry of Foreign Affairs: www.fmprc.gov.cn/eng
Germany - Federal Foreign Office: www.auswaertiges-amt.de/EN
Japan - Ministry of Foreign Affairs: www.mofa.go.jp
United Kingdom - Foreign & Commonwealth Office: www.gov.uk/government/organisations/foreign-commonwealth-office