Musopen provides free recordings, sheet music and other detailed information about classical music throughout the ages. You will find all the famous composers you've grown to love, as well as discover new favorites. The website is well-indexed for both recordings and sheet music. You can search by composer, performer, instrument, period and form. You can stream most of the music files online. You'll also be able to download to your choice of MP3 or the higher-quality, lossless FLAC file. (FLAC stands for Free Lossless Audio Codec.)
FLAC files take up more space than MP3s, but have much better sound. Another downside is that iTunes won't know how to play the file. However, you can download a free program noted below under FLAC File link to get around this iTunes glitch.
Musopen Link: https://musopen.org/music/
FLAC File Link: http://portableapps.com/apps/music_video/songbird_portable