Note that I collect but do not own these works. I just see no point in paying ~$20-50 for music sheets that are no longer copyrighted (unless you want/need the commentary). Save that money for your music lessons.
IMSLP: Petrucci Music Library (free music sheets database):
IMSLP has more music sheets (in terms of numbers as well as instruments) than Musescore.
And unlike Musescore, you don't need an account to download.