This is not exhaustive by any means. However, here are a few examples of resouces you might consult. Be creative in where you search - use not only search engines, but consider social bookmarking sites such as Diigo.
"compiled and edited by Charles J. Kappler, is an historically significant, seven volume compilation of U.S. treaties, laws and executive orders pertaining to Native American Indian tribes. The volumes cover U.S. Government treaties with Native Americans from 1778-1883 (Volume II) and U.S. laws and executive orders concerning Native Americans from 1871-1970 (Volumes I, III-VII)"
access to typescripts of interviews (1967 -1972) conducted with hundreds of Indians in Oklahoma regarding the histories and cultures of their respective nations and tribes.
over 1,100 periodicals that first began publishing between 1740 and 1900, including special interest and general magazines, literary and professional journals, children’s and women’s magazines, and many other historically-significant periodicals.
The collection currently includes 91 document projects and archives with more than 3,600 documents and 150,000 pages of additional full-text documents, and more than 2,060 primary authors.
more than 37,000 annual time series of quantitative historical information covering virtually every quantifiable dimension of American history: population, work and welfare, economic structure and performance, governance, and international relations, all from the earliest times to the present