There has to be an answer Eric. The museum must know the publisher of the poster...When I was at Michael's somebody lost two paintings from Paris, of the selling on the street corner variety. The paintings weren't worth anything but meant a whole lot to the customer. She was able to retrace her steps through various places she had gotten receipts from and pinpointed where this artist had set up shop. She contacted one of the stores near there and they were able to put her in contact with this street vendor, who of course was able to replace her one of a kind "original" art by two pieces that were uncannily the same. All that from various receipts she still had in her purse all the way to Paris, and the guy didn't even have a store front.
You have a museum where it was purchased, I bet it will take time but they will find it for you.
How about Ebay?
I'd also like to add that the missing "art" in my little tale was found several months later by somebody who was moving empty sleeves from point A to point B.
I'd also like to add that I was no longer in charge of the crew when said incident took place. I had already given up my crown, defeated........