That is, until now. I was thrilled to learn that September 30, 1955 is finally -- finally! -- available on DVD from Universal's Vault Series, via Amazon.com.
I haven't yet viewed the DVD, so I can't comment on the disc itself. What I can tell you is that the film's admirers include Janet Maslin, Dave Kehr, David Ansen, and Joe LeSeur. Once you see this remarkable, utterly unique movie, I think you will join their estimable company.
And if you're curious to know more of my opinions about the film (and details of its making -- I interviewed its stars, Richard Thomas and Lisa Blount, and its great cinematographer, Gordon Willis, among many others), be sure to order The Films of James Bridges here or here.
I'm not exaggerating when I say that my book could have been subtitled Everything You Always Wanted to Know About September 30, 1955... But Were Afraid to Ask.