The International Court of Justice (ICJ) identifies the sources of international law in the ICJ Statute, Article 38 which reads:
1. The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply:
a. international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;
b. international custom, as evidence of a general practice accepted as law;
c. the general principles of law recognized by civilized nations;
d. subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.
2. This provision shall not prejudice the power of the Court to decide a case ex aequo et bono, if the parties agree thereto.
The primary sources you will use are:
For excellent descriptions of the Sources of International Law search view the Max Planck Encyclopedia of International Law
Sources of Customary International Humanitarian Law: