The Petra Zone

Was there a Grave Robber accompaniment ever sold and if so, does anyone have a copy?

Nope, doesn't exist.

I remember years ago when Christian bookstores used to sell accompaniment tracks to popular songs, but I don't think anything like that is sold today.

You can use software like Adobe Audition (paid) or Audacity (free) to remove vocal tracks from music, but it's a bit of a hack, and the resulting sound file can end up with a lot of undesirable artifacts that can be reduced to some extent but are very difficult to eliminate entirely.