Wow, you know, I've been collecting Inuyasha cels 
since last April, but I have NEVER seen a cel of 
Sess in demon form.  I'd be amazed if any of them 
are outside of Japan, given how popular the show 
is there.  Heck, just a cel of Sess period, even 
tiny, goes for at least $80.  It's amazing.  The 
cost hasn't really gone down much, although Anime 
Museum has the best priced Sess cels I've seen 
yet (of course they're all gone T_T).  The series 
in general is expensive, even for cels of 
Inuyasha, Kagome, Sango, Miroku, Shippou, Kagura, 
you name it (except for myouga, heh heh). 

Anyway, hope that helps!

Good luck finding one!!!  ^_^

Kaerra