Right now Celga has some beatiful Mononoke Hime 
cels available.  But be prepared...Mononoke cels 
do not come cheaply.  Expect to pay into the 
$1000's for most.  And I have not seen one below 
$600.  They are absolutely beautiful.  You can 
buy repros, but they are pretty expensive as 
well, and I'm not sure if they are available any 
longer.

Another site to check out is Mandrake.  They 
recently had some wonderful cels available, 
although I am not sure what is left.  And their 
prices were about the same.  You may find some on 
Yahoo Japan as well. 

Well, that's all I've got for the moment.  If I 
can think of anything else, I will try to post.