Choral Public Domain Library, http://www.cpdl.org/wiki/index.php/Main_Page
has tons of music legally available. There are several formats on the site, but not all of them for any given title - .mus (Finale), .mid, .pdf (scores only), and .sib (Sibelious); .mus being the most common.

Myriad software, maker of Harmony Assistant, has a product to convert PDF to MusicXML that could be loaded. the .mus score can be saved from NotePad in PDF if you have the write printer driver installed (there are some available for free on the net that will print anything printable to a PDF file). But, PFDtoMusic cost $199.00. --- which I don't have. thought I'd share the info though. I've never used it so i cant say how good (or not so good) it is.

BillC