(Note that a soundtrack album for the movie Where The Buffalo Roam, for which Neil produced the soundtrack, has also never been released on CD -- but this is not really part of Neil's catalog.)
Any CD from anywhere in the world for these seven albums is NOT a legitimate release. The demand for these albums on CD have lead to a number of pirated versions, almost all of which were recorded from LPs. Some looked legitimate enough to actually be stocked as "imports" by reputable CD stores (including Tower Records). But be warned that these are in fact prirated recordings, not official releases.
Note that the Missing 6 albums were indeed actually remastered during the 90s using HDCD technology. Neil had been waiting for such a technological advancement before remastering them, and at the time the HDCD technology looked to be sufficient to meet his desires. Some of the HDCD remastered versions of the 6 had been scheduled for release several times during the 90s, but subsequently removed from the schedule each time. At one point promotional pressings for one or two of the Missing 6 were actually produced and radio spots hyping the upcoming release were distributed and recalled.
There was a point where Neil was quoted as saying that he was holding the Missing 6 "hostage" for some reason or another. But he eventually spelled out his specific objections to releasing them on CD in an interview with someone at SFX:
“The record companies have a huge problem right now. They have the DVD audio standard, and the quality is unbelievably better than the CD. It approaches what you expected from digital in the first place. It’s a wonderful standard, where the artist has control and programs the DVD so that when you put it in, it configures your system to play it back optimum for what’s on the disc. The artist decides and the format keeps changing as the artist programmed it. So you get to take advantage of all the digital information that the DVD has.”Of course that begs the question of why all the REST of his albums are out on CD (including the new albums he's released even after having withdrawn the remastered Missing 6). But regardless, it'll be great to hear the Missing 6 on DVD-audio when it does eventually hit the street...
“So what happened? We got it all together and somebody figured out how to crack it. So now they could be duplicated and the record companies wouldn’t make any money off it. But that already happened with the CD. What’s the big deal? Why not put out the quality? If people are gonna crack it and send it around on Napster or MP3, who cares?”
“So the answer to the question is, I really didn’t see these albums on CD because CDs don’t sound very good. I like the original analog masters and I didn’t want people to have bad-sounding CDs to listen to for the rest of time. I want to wait until the things are ready to be dumped into a format that I can understand and is relative to the original format.”
Footnote: American Stars'n'Bars is available commercially on cassette. None of the other albums are currently available commercially in any format. You'll have to check out the used music stores and look for the LPs or cassettes.