It's a CG show, so the only cels are repros. 
Anime Museum used to have repro OP cels for sale,
so you might want to start there. 

 Yahoo Japan sometimes has some, but the ones that
are for sale right now (a set of 2) are wicked
over-priced compared to all the other ones I've
seen on the market - which probably explains why
the seller has had to relist them for about 6
months...no one want to pay the much when they can
buy them cheaper elsewhere!

Quite a few Rubberslug members own repro cels, so
I suggest searching there to help you get an idea
of what you're looking for.  Most notable
galleries are: Shadows on White & Jenn's Cel Gallery.