Paul - if you are unsuccessful in finding professional posters of the GL1800, then consider making some of your own. Take some photos of your own Wing in settings that you find appealing and then go to a nearby Kinkos and have them setup some posters for you. Who knows you might even enjoy that better and lets face it the 1800 never takes a bad photo (at least mine never does)...
Keep an eye out on ebay, they pop up there every now & again. The yellow ones usually sell for more (because they are cooler of course!). I got one for around $25 (I think that is what I paid).
It is one of the dealer posters. Pretty cool, the one with the yellow bike with the caption "LAP AMERICA". These posters are HUGE, movie poster size. It takes up a large chunk of wall in my office. Next to it, I framed a paper map of the US and have highlited all my travels on it, to show my progress at lapping america
Not sure where else you could get them... unless you are good friends with a dealer... lol