But as a reminder, most of the acetate that copy 
stores carry tends to be 8.5x11, whereas cels 
usually have slightly different dimensions. So it 
wouldn't be exact. There are a few places that 
carry 11x17 acetate, which could be trimmed down--
-- but those places are few and far between. ^_^ 

You could also buy your own larger acetate from 
an art supplier and ask them to copy onto it, but 
some kinds of acetate will melt in a copier... so 
you need to be careful. :o)