Here are resources around the web regarding American Indians and US law:
Native American Constitution and Law Digitization Project
Housed at the University of Oklahoma Law Library, this project is a cooperative effort between the University of Oklahoma Law Center, the National Indian Law Library, and Native American tribes. The site provides access to constitutions, tribal codes, a digitized version of the Handbook of Federal Indian Law (1941) by Felix S. Cohen, documents of Indian land cessions, Indian Reorganization Act Era Constitutions and Charters, and many other resources for researchers of Native American legal issues.
Kappler's Indian Affairs: Laws and Treaties
This site contains the fully searchable digitized text of all seven volumes from the original 1903-04 U.S Government Printing Office publication. Based at Oklahoma State University Library, these volumes contain U.S. government treaties with Native Americans from 1871-1970 as well as U.S. laws and executive orders. Information can be accessed from the table of contents, through the index of each volume, or by keyword search.
See also the Library's U.S. Law Research Guide online guide.