According to Restatement of the Law (Third), the Foreign Relations of the United States, section 102(4), "General principles common to the major legal systems, even if not incorporated or reflected in customary law or international agreement, may be invoked as supplementary rules of international law where appropriate."
General principles are legal concepts so fundamental that they can be found in legal systems throughout the world (e.g. 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.
In researching general principles of international law, you will be looking generally for the laws and customs of the world’s major legal systems.
Since this task is necessarily overbroad, start by consulting background sources on general international law. Then look for international decisions that cite general principles and domestic legislation as evidence of custom.
Principles of Public International Law by 
International Law Frameworks, 4th by 
General Principles of Law - the Role of the Judiciary by 
General Principles of Law in the Decisions of International Criminal Courts and Tribunals by 