Keywords: Indigenous peoples of the Americas myths mythology
Project Gutenberg contains many books on Mythology of the Americas which are downloadable in epub or Kindle formats, as well as readable online.
If searching for mythology of Indigenous People of North or South America, using specific tribe names along with mythology or folklore might give better results. For instance, searching JSTOR database with mythology and narrowing the subject to folklore gives over 4,000 results. If you add in Pawnee and Crow as a keyword, you can narrow it to 50 results. Using the term Native American or Indigenous People as keywords does not narrow down the results as well.
If you are looking for an archetype in these cultures add that into your search as well.