http://aimeemajor.com/anim/lsupply.html

here, any of these places should be able to 
sell you not only blank cels in lots of sizes and 
formats (even frosted), plus they have cel 
paints, etc...

;} or if you want to attempt actually animating, 
this is my animation technique site:
http://aimeemajor.com/anim/
I appologize I havent updated it in a long time, 
but it still has lots of information.