Ebay's gonna be your best bet, especially for Jem 
cels. It's been a long while since any Jem cels 
were available at animation art dealers. 

M.A.S.K. is a tough one. DIC never distributed 
MASK cels the way they did with some of their 
better known series, and most animation art 
dealers couldn't recognize a MASK cel if they had 
one. It's pretty frustrating, I managed to get a 
few of MASK cels, but have been searching for 
Pole Position cels. Unfortunately, DIC destroyed 
those cels a long time ago, and only a few remain 
in the hands of the people who worked on the 
show. 

Kett