Cels Forum

Well, there were some original production sketches of 
Deedlit on ebay- most people display production sketches
next to their cels because most cels come with them.
If you are looking for original artwork insofar as 
artwork done by the creator of a series or the manga artist,
I'm afraid you're probably out of luck unless you go to
a convention, and even then the con would probably have to
have that person for a guest (though not necessarily).  
Even if all those factors are there, you still might not
find anything.  Some of these things do go for sale on 
ebay, but obviously as you mentioned you would have to 
be pretty careful- I would probably feel okay doing that
for a series I like, I think I have a pretty good eye,
usually, when it's series/art I know well.