Lewisham Ukulele Club provides a list of the most popular ukulele songsheet resources.
Here’s some of our favourites . . .
Michael Madden’s, Collection of Old Tunes for Ukulele (all copyright free for USA & AUS) (https://archive.org/details/CollectionOfOldTunesForUkulele)
Quayside Uke‘s collection of over twenty songbooks (right-side panel of their home page)