Many good sources of music scores are availabe online if you know where to find them. Don't just go with the cheap ones, but the more reliable and authoritative ones.
Digitized music of orchestral scores and parts used and archived by the NY Philharmonic; Will see bowings and markings from legendary conductors such as Leonard Bernstein and Bruno Walter.
Digitization project of the Bavarian State Library and others in Germany. Provides free access to manuscripts of important composers such as Handel, Beethoven, etc.