It's been one of my grails, and i'd sure like to find one. Someone's selling an A4 print of it on Ebay.. but it's probably just a color inkjet print of some JPG they found online.
While I haven't seen it on a poster, I do own an officially-licensed t-shirt that features this image. If I'm not mistaken, I got it around the time the first live action movie came out. Yeah, just checked it out - copyright info is Hasbro 2007. It being official artwork, there's a decent chance that it was printed on a poster, but I'm not sure. I will say that the art on the image above is much cleaner/crisper than the one on my shirt. Good luck on your quest for your grail. MIKE engledogg
Jay jays here in new zealand had a gaint one a few years ago as part of the 25th tf aniversery so they are out there When i say gaint, i mean im 2 meters tall and it was bigger than me
Found this on Etsy. Transformers The Movie G1 Autobot vs Decepticon Rodimus Prime Unicron Poster Canvas Print #113
"Please note this is not a high resolution print due to the design of the graphics." I'm afraid that's another person selling the jpeg they found online.