I used to have tons of album flats. I really miss them, but they were just beat to hell after three or four years.
You might try ebay or something like that for "album flats" which were cardstock 12x12" reproductions of the album artwork. Almost all the ones I had were produced in the CD era, so don't think they were an anachronism from the 80's or anything.
go to
http://skinniesrecords.com and inquire about individual bands. He just may have what you are looking for. I wouldn't bet on hardly anything that was not on a major label to be around though. In my experience the indie metal labels just didn't bother with them. They were mostly part of the Concrete Corner promotional scheme.
There are probably tons of Dream Theater ones left. I had everything up to LIVEtime before and he had plenty of them a few years ago. People usually didn't bother looking at what he had, so that sat for a long time until someone like me that went through everything found them. I paid a dollar each for them at the time, but don't be shocked if the price goes up due to him having to go through the thousands he has to help you out. I'm not guaranteeing anything. He could have what you want and is willing to send them to you fairly cheap or he could tell you he doesn't know what you are talking about.