"Terror from the Year 5000" (episode 807) has not yet been released on an official DVD. Neither of the other titles you mention were done on MST (unless under an alternate title--a lot of these cheapo flicks have been released under several titles, and sometimes the title had to be changed just to get the rights for MST).

If you poke around a bit it's fairly easy to find dealers who sell DVDs of episodes that haven't been officially released. The quality on these is usually excellent, and several of the dealers also have "bonus" disks of outtakes, behind the scenes stuff, commercials, specials, talk show appearances....etc. etc. The reputable dealers won't sell episodes that have seen an actual commercial release, but many episodes are unlikely to ever be put out officially for various reasons.

Shout's "Volume XVII" boxset actually came out just today, and of its four episodes at least one is a full-bore classic: "The Final Sacrifice," featuring that Canadian answer to Indiana Jones, Zap Rowsdauer. Between all the official releases and various dealer buys I've got 150 episodes on DVD; by my reckoning I need 48 more, but that's counting all the KTMA episodes, three of which are not known to exist in any fan copies.

I've tried the rifftrax mp3 thing a few times, but haven't really found it to be worth the effort. They do mostly recent releases (and a scattering of classics--would you believe they've done "Casablanca"?), and it's just not worth sitting through, say "Transformers" to hear the jokes. I think riffing works much better with older/worse material. They have had some good guests on some tracks, like Weird Al and Joel McHale. I much prefer the older movies they can actually put out on DVD, like "Planet of the Dinosaurs." I think the best thing Rifftrax has done, though, is their volumes of shorts, several of which are available through Amazon or directly through the rifftrax site. The shorts were often the best thing in MST, and RT keeps up that tradition proudly.

If you're thinking of becoming a riffing completist, one thing you'll want to look for are the "Film Crew" DVDs, four releases of truly awful, cheesy films with riffing by Mike, Bill Corbett and Kevin Murphy. They made these before they became the regular RT crew, and who knows how long they'll be in print.

There are some fan/homage efforts that are also worth seeking out, including especially the Incognito Cinema Warriors and Josh Way, who you can sample on Youtube--look for "Fun With Shorts."

For my money Cinematic Titanic (Joel, Trace Beaulieu, Mary Jo Pehl, Frank Conniff, and Josh Weinstein) has been much more consistently funny than Rifftrax, though they have many fewer releases--seven studio DVDs and two DVDs of their live performances. The movies they use are just unbelievably cheesy and bad--when I say that Roger Corman's "The Wasp Woman" is the best of the bunch, it should give you some idea. You can buy their DVDs directly through the website, and they also tour live.

Not that anybody asked, but my favorite eps of MST, in no particular order, would be:

1006--Boggy Creek II
903--Puma Man
820--Space Mutiny
208--The Lost Continent
303--Pod People
421--Monster-a-Go-Go
512--Mitchell
618--High School Big Shot
424--Manos: The Hands of Fate
815--Agent for H.A.R.M.

Can you tell I'm a fan?