I have not particularly seen cels of this
character but I would try:

1) www.cels.org - Many dealer cel sites to look
through

2) www.rubberslug.com which is a cel site that
individuals host their cels for view and sale.  If
you see something you like you could register for
Free and contact the owner.  

3) This way is more complex but can be ultra
rewarding.  You could try www.rinkya.com for
buying cels straight from Japan a-la auction (like
Ebay).  You would have to commission someone to
make a purchase for you (their people are in the
US and Japan who translate for you) but they
require a percentage based fee.  Another 3rd party
is www.Celga.com.  Those are great sources to find
rare, hard to find anime cels as the online
inventory always updates.  They could also point
you in the direction of a Japanese cel site and
help you purchase right from them without bidding.
 Of course you may have to click on links you
cannot read but that's part of the fun ^_^  You
can go to www.xe.com for a simple universal
currency converter to check prices.

I hope that helps!
     John