I collect, and occasionally write, traditional tunes in various traditions. Most of the music below is in an open-source format called abc. If you don’t know abc, check it out – it’s great for traditional musicians!
Old Dutch music
Once upon a time the excellent blog Lusthof der Muziek offered an incredible amount of sheet music of old Dutch music (dance tunes and song melodies), until Dropbox changed its policy. The owner of that blog was so kind to send me his abc files so at least I can make those available. Most of it is traditional, but be aware that putting it in abc and other formats is the result of painstaking work of countless volunteers. Download the files here.