Researching customary law is as complex as it is essential. Some useful (but by no means all) sources include: digests of state practice (for those nations that keep them); domestic/municipal law; and international law yearbooks. Actions by international governmental organizations (such as the U.N.) may also serve as evidence of customary law. For these actions, see the United Nations/IGOs section of this guide.
These are some good online and print guides on researching customary law. Where good digital sources for customary law research are available, the online guides will take you to them.
These are just a few of the many other useful sources for researching United States state practice in international law. For more options, see "Advice on Researching Customary Law," above.
Here are some good places to begin your search for state practice of non-U.S. nations. For more options, see "Advice on Researching Customary Law," above.
Domestic laws (including constitutions) can provide evidence of customary law, as well as of "general principles." Here are a few places to start.