From Sources of International Law: An Introduction by Professor Christopher Greenwood:
"Customary law is not a written source. A rule of customary law, e.g., requiring States to grant immunity to a visiting Head of State, is said to have two elements. First, there must be widespread and consistent State practice – ie States must, in general, have a practice of according immunity to a visiting Head of State. Secondly, there has to be what is called “opinio juris”, usually translated as “a belief in legal obligation; ie States must accord immunity because they believe they have a legal duty to do so."
While Customary international law is not a written source, one can find documentation of customary law in state papers, diplomatic correspondence, national legislation, executive decisions, judicial decisions of both national and international courts, yearbooks, and in scholarly works. See Duke International Legal Research Tutorial
U.S. State Department
Documents of Intergovernmental Associations
Included in the list of sources of international law in Article 38 of the International Court of Justice Statute are “general principles of law recognized by civilized nations” (i.e. general principles of fairness and justice which are applied universally in legal systems around the world). Examples of these general principles of law are laches, good faith, res judicata, and the impartiality of judges. International tribunals rely on these principles when they cannot find authority in other sources of international law.
These general principles of law can be found in decisions of international tribunals and national courts; references to them may also be found in the teachings of the “most highly qualified publicists” (i.e., eminent international law scholars). In fact, Article 38 includes judicial decisions (of both international and municipal tribunals) and scholarly writings as “subsidiary means for the determination of rules of law;” in other words, these are not authorities, rather they are evidence of the sources of international law.