The only place I recall ever seeing one is in Elite II Frontier. If you have that game (You could probably pick it up second hand for a small amount) go to the galaxy map and zoom right out. Then you'll have to find a way of grabbing the screenshot. Other than that I can't suggest anything you haven't already tried (probably)
The BBC recently did a programme about astronomy with lots of snazzy CGI. I just had a look at the website and couldn't find any Milky Way images. Didn't look too hard though. Some interesting stuff there anyway, mind you.
One other possibilty - find the website for the Milky Way chocolate bar (Nestle I think) and maybe (just maybe) they'll have something. That is getting a bit desperate though.