International custom requires both state practice and proof of opinio juris sive necessitatis ie that the state had felt legally obliged to act in that way.
To find commentary on SOLO use subject search
Customary law, International
Custom as a Source of Law by 
Customary international law : a new theory with practical applications by 
The Changing Nature of Customary International Law by 
The Discourse on Customary International Law by Sources to establish a state's practice in international law
Try the official website of that government’s Ministry of Foreign Affairs or Ministry of External Affairs etc.
Many states publish an annuaire, Jahrbuck or yearbook on international law. (There is no unanimity with yearbook, year book or year-book).
Sources of State Practice in International Law by Vienna Convention on the Law of Treaties (1969)
art. 53: A treaty is void if, at the time of its conclusion, it conflicts with a peremptory norm of general international law. For the purposes of the present Convention, a peremptory norm of general international law is a norm accepted and recognized by the international community of States as a whole as a norm from which no derogation is permitted and which can be modified only by a subsequent norm of general international law having the same character.
art. 66 (a) disputes on such a norm's existence can be submitted to ICJ.
The International Law Commission's Draft Conclusions on Peremptory Norms by Jus cogens (International law) is a specific subject search term to try in SOLO
Peremptory Norms in International Law by 
Peremptory Norms of General International Law (Jus Cogens) : disquisitions and disputations by 
Jus Cogens: international law and social contract by 
Peremptory International Law - Jus Cogens by