There are sites that sell MIDI song files-- including Yamaha MusicSoft, of course-- but I haven't bought anything from any of them, so I have no personal experience on which to judge the quality of their products. If you want to look for alternate sources of specific songs, your best bet might be to search for the name of the song plus "midi," e.g.:
beatles come together midi
You might find several free MIDI files that way, as well as commercial files. However, you'll probably still end up wanting to revoice the song file for your given keyboard model no matter where you bought it from, because the file will most likely use GM voices, or-- if it was aimed at Yamaha keyboards-- XG voices and features, so you'll most likely want to tweak the voices in the file, and possibly edit it in other ways (e.g., adjusting the volume or velocity of the notes, adding Control Change events, etc.).
Oh, man, I just realized that my previous response was about revoicing style files rather than song files! Although "Mix Master" does work with either type of file, I should have recommended "MIDI Revoicer" instead of "Style Revoicer":
http://www.jososoft.dk/yamaha/software/revmidi/index.htm
Sorry about that!