Customary international law consists of rules derived from the consistent conduct of States acting out of the belief that the law is required to act that way. State practice is the key to understanding this type of law.
The sources below give some background information and advice about how to locate and identify custom and state practice as sources of international law.
This database of customary international humanitarian law is divided up into two parts. The two parts include the following:
1) a section on customary rules that may be applicable in international and non-international armed conflict, and
2) a section on practice by states as well as information on international organizations, conferences, and judicial and quasi-judicial bodies.
This database provides documentation and brief commentaries concerning the implementation of international humanitarian law (IHL) at the national level. The database includes the texts of legal measures adopted by States. Such measures may take a variety of forms, including constitutional provisions, specific laws (e.g. Geneva Conventions Acts), parts of more general laws (e.g. criminal codes), regulations and decrees. Relevant decisions of national courts are also included when available.
NYU's Globalex, including this research guide, is another useful source for finding resources on customary international law.
Sources of State Practice in International Law by 
This link opens in a new windowYearbooks can be valuable sources for identifying state practice. HeinOnline’s Foreign & International Law Resources Database has a good selection of these for various countries and regions.
This Restatement of the Law (Third), the Foreign Relations of the United States (Law Library KF395.A2 F6 1986) discusses how the United States interprets international law. It is sometimes cited for international law principles including customary law. It can also be found on Lexis and Westlaw.