37 Ghosts and 38 Ghosts multitracks were part of the multitracks that came in the $300 and $75 versions of Ghosts.
So far, fans have re-constructed the tracks by laying the multitracks together.
MP3s, FLACs and multitracks can be found at ETS, because I'm too lazy to link to them indivitually.
here.
The Nine Inch Nails Guild