The following online research guides are great places to start. Also see the books listed in the box below.
Law Library of Congress' Indians of North America Guide
Includes links to legal texts, commentary, agencies, and other guides.
Basic Indian Law Research Tips - Tribal Law
David Selden, National Tribal Law LIbrary at the Native American Rights Fund, Jan. 2012 [5-page PDF].
Indian Law and Tribal Law & Government
The University of Montana School of Law Library Research Tool.
Tribal Law Gateway (National Indian Law Library)
Find materials by tribe (includes an alphabetical list of tribes) or by type of document.
Native American Law & Legal Sources
University of Wisconsin Law Library.