Try a Warner Bros. store. They're mostly in 
malls, although the ones near me have closed in 
the past year or two. I remember paging through 
big binders from shows like Batman, Pinky & The 
Brain, Tiny Toons, Animaniacs, etc, that Warner 
Bros. puts out, and they had production cels and 
sericels for sale. All the stores I've been to 
have one corner dedicated to sericels, mostly; 
it's just a matter of asking the salesperson (a) 
if they have any P&B cels in-stock that aren't on 
display, or (b) if they can obtain P&B cels.

Granted, though, they tend to be pricier, partly 
because there's a domestic audience who likes 
animation art, even if they know nothing about 
cels; and partly because operating out of a mall, 
it's always more expensive that way. ^_^

Good luck!
-Cres