FAQAdd to FAQ (Coming Soon)
Nowhere. It just does not exist. The closest we have to David Lynch's original vision is the theatrical release. Some people sometimes refer to the extended TV cut as the "director's cut" (and some even dare to offer copies of it in various auction sites under that label), but that couldn't be further removed from reality, as that version was rejected by Lynch to the point of deleting his name from the credits. A legal DVD of Dune: The Director's Cut exists, but that's for the 2000 Sci-Fi Channel miniseries, not this film.
However, the cast and crew of Lynch's Dune have confirmed that Lynch showed them an 'assembly cut' of the film shortly after principal photography wrapped. Although the film had yet to go through post-production, the reaction was very positive. This cut has not been seen since (as is the case with most assembly cuts).
There is also a fan version available, that attempts to get closer to both the novel and David Lynch's original vision: http://www.fanedit.org/ifdb/412-dune-the-alternative-edition-redux Edit (Coming Soon)
It's "Backyard" by Emmett Chapman, included on his album Parallel Galaxy. Chapman was also the creator of the instrument (known as the Chapman Stick) that Gurney plays, repainted and with an added lower part to represent the Baliset described by Frank Herbert in the original novel. Edit (Coming Soon)
Both versions contain footage that's missing in the other one, but the Theatrical Version lacks way more footage. Furthermore several scenes have been removed and reinserted afterwards in the TV Version, some of them are edited differently. But these differences aren't mentioned in this comparison because it would break the mold. David Lychn's version is the Theatrical Version btw. He wasn't involved in the TV Version and ordered that his name in the opening credits was going to be replaced by Alan Smithee . The main difference in the two versions is the fact that the TV Version has been split up to smaller episodes. As a result of that, the movie doesn't look like a motion picture anymore but like a big TV production. Edit (Coming Soon)