Be careful not to box yourself into a concrete search term for your topic. Think of synonyms for your subject - broader terms, related terms, and narrower terms. Using this strategy will guarantee a wider selection of materials. Be aware, too, that some search terms may be uncomfortable to think or say out loud - but could yield possible results.
Possible search terms: Indigenous, American Indians, Native Americans, Aboriginal, specific names of tribes and bands (e.g., Choctaw, Tlingit, Miami)