Archives.gov is another very valuable online research portal that was started by congress in 1934. The National Archives was started in order to preserve records of the United States government.
Another easy to use government resource, you can simply type in your search inquiry in the top left search box and then click the Search button.
I have searched for: "Mississippi Census":
From here, I chose BIA Records: Mississippi | National Archives.
The National Archives houses information about the state census records for Native American tribes and is broken down by states. Here is the information from the Bureau of Indian Affairs (BIA) for the state of Mississippi.
Other records you can find at Archives.gov include military records, genealogical records and information and see online copies of many historical documents such as the Declaration of Independence.