You supposedly cant download them because they are illegal but thats the only way you can get them
it only comeswith 3 songs
Ideally what you are supposed to do is download the 'frets' and then get the music from your own collection and put it in the game with it. Obviously that is quite a bit of extra work, so I think if you own the songs it is fine to get them, but chances are you won't own all the songs if you get a big pack.
So as far as this thread is concerned, don't post any direct links or strong suggestions to downloads of non-legal songs (there are quite a few legal songs linked to by the official site for the game), and you will be fine.