I was lucky enough to get the entire Wikifonia database of 6,675 lead sheets in mxl format. This is over 300 more songs than available on another site. (Wikifonia is no longer in existence.) The file is only about 35mb. Here is the link:http://www.synthzone.com/files/Wikifonia/Wikifonia.zip
You will need to download the free software Musescore to be able to view, transpose, edit, print pdf's etc. You will also be able to write your own songs with Musescore.http://musescore.org/
Musescore also has some songs that can be downloaded.