The University of South Carolina Music Library has an extensive collection of over 10,000 pieces of popular, sacred, and classical sheet music that can be easily searched, due to the digital sheet music project on the School of Music website. Note that this includes popular music as well!
You can search by song title, composer, lyricist, etc. For music in the public domain, they provide full images of the music. Great way to find that older tune you’ve been looking for, save a little cash, or discover older gems that might round out your cabaret show. Enjoy!