Hi,
  Here is the web address for Blondies (formerly 
American-Plas Tech) where you can buy good 
quality polypropylene bags for your cels.

www.blondiesnet.com

Also, if you need Mylar for your sketches (this 
is the #1 preservation for paper goods/ also 
used by the Library of Congress)  Don't use 
Mylar on cels.  You can get it though Blondies 
or you can get it directly from E Gerbers

E Gerber
http://www.egerber.com/items/mylites2.htm

Hope this helps,
Linda