Willa Cather Archive
Though it is already used in this Lesson Plan, the Willa Cather Archive from the University of Nebraska, Lincoln is an excellent resource containing Cather short stories, scholarship, photographs, and more.
Willa Cather's Prairie
https://www.newyorker.com/magazine/2017/10/02/a-walk-in-willa-cathers-prairie
A thorough article from the New Yorker on the Nebraska prairie's influence on Cather's literature
American Masters: Willa Cather
http://www.pbs.org/wnet/americanmasters/willa-cather-about-willa-cather/549/
PBS's American Masters series page on Cather, including videos, articles, and classroom resources