Allposters is a division of Art.com. Art.com being the parent company will often have things that Allposters doesn't, but not the other way around.
Just a wild guess about availability elsewhere...since it is available in so many sizes, and the pricing is pretty steep for your basic 4 color offset non-vintage poster, my guess is that Art.com has the publishing rights to this one and it is being printed in house on an ink jet printer. The short turn around is the other clue.
The heavy discount on the one size may be from an over run from a previous printing, or they bought up another distributors/publishers remaining inventory. If that's the size you want, you might as well get it from them unless you just want to spend several hours searching for the fun of it. I've already checked the major consolidators that I deal with and none of them have it on their site.
