Those are the bags spinizuey are talking about. 
They are really stiff and shiny.  I think they are
the best presentation-wise.  The only caveat is
you should buy acid-free backing tissue to back
your cels in these bags as they do conduct a lot
of static.  This can cause the paint to bond to
the bag.  :(  I've never had the paint stick to
the tissue, but if it did, it's nothing a little
water can fix.

http://www.bagsunlimited.com
http://www.preservesmart.com/