> So the most likely answer to "where's my background" is:
> With another cel from the same sequence.

But the original poster said it was an A1 END cel -
which means that it was the only cel used in that sequence.

The studio could have kept background to reuse it, or 
on rare occasions it was in the contract not to release
backgrounds.  With a Pokemon cel I would speculate that
the background was probably reused for another scene.