The National Indian Law Library (NILL) posts a substantial collection of tribal constitutions, codes, treaties, court opinions and other primary and secondary legal materials.
Lexis offers a strong collection of Native American legal materials. The collection includes treaties, legal codes for many tribes, tribal court opinions, Bureau of Indian Affairs, Presidential orders, Cohen's Handbook of Federal Indian Law, news and gambling-related materials.
Westlaw's Native American legal materials can be found by clicking on the "Directory" link at the top of the screen and then choosing "Topical Practice Areas > Native American Law."
VersusLaw offers a database of judicial opinions from tribal courts. (Note: Although VersusLaw is available only by subscription, you can get a 1-day subscription for a modest fee.)
Many of the leading U.S.-Indian treaties are reprinted in Documents of American Indian Diplomacy: Treaties, Agreements, and Conventions, 1775-1979.
The Encyclopedia of United States Indian Policy and Law discusses U.S. laws related to Native Americans (CQ Press).
The Rights of Indians and Tribes (Oxford University Press) is a popular and helpful 1-volume discussion of U.S. Federal Indian law.
The United States Tribal Courts Directory (W.S. Hein) contains contact information and a brief discussion of each court.
For more sources: The Appendix to Bonnie Shucha's
'Whatever Tribal Precedent There May Be': The (Un)Availability of Tribal Law "lists existing publicly available tribal law collections, both historical and current."