The Star Wars trilogy special edition - behind the scenes It was split into three parts for the VHS release but this is the complete half hour version from the laserdisc with a bit of extra stuff! Best quality version I can find.
The Mythology of Star Wars Filmed around the time of the release of Episode I George Lucas sits down and talks for an hour about everything Star Wars! Personally, I love this - it’s a really, really good interview. Not the best quality version available so it might be worth doing a bit more searching.
When Star Wars Ruled The World A fun VH1 collection of interviews from the 90’s which is fluff, but still interesting! (Probably not the best quality version out there.)
Star Wars - The Magic and The Mystery Another fluffy fity minutes (presented by Howie Long!) but it’s well put together and worth your time I think.
From Star Wars to Star Wars- The Story of Industrial Light & Magic Samuel L. Jackson presents a brief overview of ILM from '77 to '99. Pretty low quality video but the best I can find.
A more recent documentary is The Galaxy Britain Built which aired at the end of last year about the UK’s contribution to Star Wars. I’m patriotically biased but it’s a really interesting watch and has a lot of behind the scenes staff (and Gareth Edwards) sharing their stories.
Ralph McQuarrie: Tribute to a Master Shown at Star Wars Celebration a few years back and then split into five parts on starwars.com, this version edits them back together - it’s in HD (although not the highest quality possible.)
I think starwars.com has the best quality version of From Star Wars To Jedi - The Making Of A Saga
As others have said, some of what you’re after is on the Prequel Trilogy dvds. Whatever your opinion on the actual movies all three of those releases are FULL of great bonus material which is definitely worth buying despite being in old fashioned standard definition 😉
Also, I must mention Jamie Benning. He’s made several outstanding (and very professional) documentaries which are on his Vimeo channel, also some other non-Star Wars stuff you might find of interest. Highly recommended!