Can't find them anywhere on my drives, I guess I never actually downloaded them. I do have the 1978 reissue trailer.
Archive.org has the page, but the download links don't work. :(
Weird that the specific particular blog entry about the THX ads has vanished from the official site. I dug pretty far back into their archives, and it's gone.
If all else fails, we just have to nab that pesky record next time it pops up on Ebay.