By now I'm pretty sure it is somewhere in here, too. I haven't seen it, but really finding something that is new isn't easy on the internet.^^
What might be really interesting is what I discovered on ebay. The product info says it is a cell from 1988, coming from the Sullivan Bluth Studios, including the handpainted background. It looks like this:

If I'm not totally missing something here, we never saw this background in the movie. Of course there are some possibilities here:
1: It's fake.
If someone faked it THIS perfect, it's worth the price anyway (84 USD). xD
The seller has 100% positive feedback btw. And the look of the cels matches perfect to those I got some time ago (referring to the numbers on them and stuff).
2: It could be that those are cels used in the scenes we all know, smashed on a fake background that got printed somehow. Again, the seller has the 100% rating. According to the description, it actually is a handpainted background. You can even see that the paint is a bit smeared at the edges of the painting
3: It could be from another movie.
I don't think any other movie besides the first one used cel animation. They might've drawn backgrounds per hand, but again, 100% rating for the seller and it clearly says when and where it was made.
4: It actually is a background plus two cels from a scene that didn't make it. If that's true, from the looks of it it seems to belong to the great valley.
The link to the cels+background is here:
click.
Somehow I really wonder how it happened that these cels are now in the possessions and sold by some random guys all over the world apparently...xD