Tribal law is the law developed by each tribe via constitutions, codes, and cases.
People used to researching federal or state law will often be challenged researching tribal law, because there is no database service or print system that covers all tribal sources. Researchers must check multiple sources and accept the fact that their research might still have gaps. See Bonnie Shucha, 'Whatever Tribal Precedent There May Be': The (Un)Availability of Tribal Law, 106 Law Libr. J. 199 (2014).