Primary Sources of Public International Law are treaties, international custom, and general principles of law. Judicial decisions and the teachings of the most highly qualified publicists of the various nations are secondary sources. In a civil law country, normally a constitution, statutes, regulations, rules, ordinances and decrees are primary sourcer, and cases are secondary; on the other hand, in a common law country, cases have primary authority.
In order to find international custom, please begin with Researching Customary International Law, State Practice and the Pronouncements of States regarding International Law by Silke Sahl.
Researching foreing laws is introduced by my another research guide, Foreign Law Research in this Libguides database.
International custom relating to the rights of indigenous people has not been established firmly. Sui generis standing of indigenous people and the land rights of indigenous peoples are alleged to become international custom. There are treaties like CERD, and soft law such as the United Nations Declaration on the Rights of Indigenous Peoples and Agenda 21 which are helpful for looking further into potential customary law. Generally, the Univeral Declaration of Human Rights still can apply to this case.