Good question, Getlo.
1. A ten disc dvd set of all footage shot for Elvis On Tour.
2. 2 disc dvd set of the 1977 CBS footage.
3. Pied Piper footage.
4. Any 1970 Astrodome soundboard
5. DVD release of a selection of Ep's private films, featuring Priscilla et al.
Don't know if the first two fit the bill as being 'Grail like' ie, mythological...........though I do think they will be harder to find.