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.  

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.