Here for the first time ever, to the best of my knowledge, is a reproduction of the Liber Usualis that can be downloaded one week at a time. You should probably still buy a real one, but this allows you to start singing propers while the hard copy's in the mail, without having to download the entire scanned Liber from Musica Sacra.
http://www.institute-christ-king.org/latin-mass-resources/sacred-music/
Enjoy.
