Many of these sources were originally collected by other librarians--in particular, the library staff at Oberlin Conservatory have done yeoman's work for us all
https://songofamerica.net/
American song database including resource lists, lessons plans, searchable database of composer, poets, writers, and songs
https://spirituals-database.com/
the creator of this website has a book deal, which has resulted in some content being unavailable, but is still much information and many links on this site
https://www.songhelix.com/
Provides singers and teachers a tool for finding related repertoire and assist with creative programming through novel associations
https://www.sparksandwirycries.org/magazine
Explore example recitals on various topics. Also includes articles written by creators and practitioners of song which include many repertoire ideas