The majority of federal acts related to American Indians are located in Title 25 of the U.S. Code (link to free version of U.S. Code available from Cornell's Legal Information Institute).  As a reminder, annotated versions of the U.S. Code are available on Lexis and Westlaw.  You can use the annotations, Citing References (Westlaw) and Shepard's Reports (Lexis) to find cases, administrative materials and secondary sources related to specific sections of the U.S. Code.  Historical editions of the U.S. Code are available via HeinOnline.

The Indigenous Peoples of the Americas Library from Hein also contains compilations of legislative histories for the most important federal acts including: American Indian Religious Freedom Act, Indian Child Welfare Act, Indian Reorganization Act (and amendments), Indian Health Care Improvement Act, Tribal Law and Order Act,  Tribal Self-Governance Amendments of 2000 and others.

Apart from the compiled legislative histories related to specific Acts of Congress, a number of additional Congressional documents (hearings, reports, miscellaneous House & Senate documents) exist from the19th Century as the county's territory was expanding.   The authoritative guide to documents from this period is Steven L. Johnson’s book, Guide to American Indian Documents in the Congressional Serial Set: 1817-1899 (available in the law library at MON KF8201.A1 J63). 

The Oklahoma Department of Libraries and the University of Oklahoma Donald E Pray Law Library scanned the documents from the Serial Set that are referenced in this guide and now provide free, online access to this content through their database:

American Indian and Alaskan Native Documents in the Congressional Serial Set: 1817-1899

Additional Congressional materials may be accessed through ProQuest Congressional.

This database allows for full-text searching of bills, hearings, reports, prints, etc.  When searching for testimony in Congressional hearings, a helpful field to use in the advanced search option is "Witness Affiliation." American Indians testifying before Congress will usually have their affiliation listed as "XYZ Tribe" so try searching for tribe in this field.