The Music Room is a commercial site offering some scores for free and a sample before you buy policy. Music Scores offers scorch sheet music to sample and then print. Sheet Music Plus has a huge collection of Sheet Music to buy. Virtual Sheet Music has quite a lot of classical works and some free.
Any Score similarly has classical and other sheet music and some for free.
Allegro Assai is a stylish Italian sheet music site. Classical Enthusiast has some useful information and links relating to Classical Music. It is also worth trying the following sites which also provide classical music in various forms. EMusic has a variety of different MP3 downloads. Rainmusic has MP3 and Sheet Music.