I will update this post with high-quality download links or sources here if you guys can post them in the thread.
-http://www.youtube.com/user/Homestar14/videos
This guy uploads song vocals, background vocals, instrumentals, and background music from the show in high quality. Has download links with multiple audio formats in the video descriptions.
-http://www.mlptf2mods.com/tutorials/resource-materials
Downloads for the voices for every episode. This website only offers the middle audio channel from the episodes, so it does not include backing vocals and the chorus for certain songs.
-http://www.everyponysings.com/p/music.html
Sheet music, chords, and guitar tabs for the songs.
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Please post links here to high quality downloads for all the vocals of the songs from the show - not the Equestria Girls movies.
Some of the acappellas can be found on youtube, but they're usually with no download links and at lower quality. We should put download links here all in one place where they can easily be collected and downloaded for people who wanna make remixes.