Plastic pockets for cel portfolios?

For sketches & backgrounds, I use the polypropelene B4 & A3 folders that I can find in the Japanese stores around here. They're smaller and a much more convient way to store paper items than the Itoya is. Depending on the size of binder, there are anywhere from 10-20 pages to store items in. With a 20 page B4 binder, that's good for 40 backgrounds.

I have some extra A3 binders, if anyone is interested.