I use 3 mil poly bags from Bagsunlimited.com. They have a 
huge selection of bags for all sorts of collectors. They don't 
have any bags they they say are for cels, but they have bags 
for everything from postcards to posters and everything in 
between.

I use 3 mil poly bags and the .04 backing boards, kept in 
acid free museum boxes. It's a good way of keeping cels 
protected, and sorted out if you have a particularly large 
collection.