In general terms, general principles of law can be a gap filler in international law that is generally universally applied and may refer to procedural, substantive, or interpretive matters. Examples include res judicata, and the impartiality of judges.
There's much scholarly debate on exactly what general principles are and how to locate them. Currently, the International Law Commission is studying this topic but has not yet issued draft conclusions.
General principles can most readily be derived from discussion in secondary sources. Consult the Selected Books Discussing General Principles of Law section for books focusing on general principles. In addition, check for a Max Planck Encyclopedia entry and search for journal articles on your topic. You may come across decisions of international tribunals delineating particular general principles in your research, but it's generally more efficient to learn about those decisions through secondary sources than through researching cases directly.
You can also conduct independent foreign law research to determine which individual states follow a principle in order to show that a large contingent of states follow the same principles, but this will be a time-intensive and challenging endeavor. Consult the Foreign Law Research guide for tips on foreign law research.