I think the thin, onion skin like paper you're 
thinking about is rice paper. As for where to 
find it, I live near Boston and sometimes go to 
Chinatown to buy nice rice paper for calligraphy. 
(I can sometimes also find it at Sasuga, a 
Japanese bookstore where I get most of my origami 
paper.) But I don't have any idea if that is acid 
free, so I don't know if it would be good for 
using with a cel. Try asking in a big art supply 
store.

Many Sharp Smiles,
--Drac