http://www.iconusa.com/Onlinestore/sleeving.html

I've bought 200 11"x14" sleeves from them so 
far. . .about to run out again.  (It can also fit 
standard BGs as well).  As I said before, these 
sleeves will perfectly fit in Itoya 11"x14" books 
(and obviously larger books).  The sleeve in the 
cel book pocket would also create another space 
to put sketches and timing sheets behind the cel.

Besides replacing warped bags (which is basically 
all the cels that I've been getting recently), 
they're also useful as protection between layers 
of cels and between the cel and background.  Just 
cut up the sleeves and stick the resulting 
polypropylene sheets between layers.

I had to do that with a multi-layer Washu w/ BG 
cel of mine.  The previous owner had put 
protecting sheets in between the layers, but by 
the time I got to the cel, the protecting sheets 
were extremely warped and not doing much 
protecting at all.  So, I just simply cut up the 
sleeves and placed a sheet between each layer and 
the background.  The sleeves are big enough to 
cover standard sized BG's.  Now I have a cel 
setup with 3 layers and a BG that I don't have to 
worry about sticking together.

IconUSA sleeves are 3.6mm thick, so they won't 
warp for a long time as well.