Cels Forum

Go to a scrapbook supply store.  You can get acid-free
colored or white cardstock or paper there to put behind your 
cels. 
These scrapbook shops also sell acid-free 10"x10" (more or less)
page protectors that are specially made for scrapbooks.  I put 
my cels in those and then into the Itoya type portfolios.  They
work perfectly and I don't have to hunt around for those elusive 
cel bags.  The only drawback about these page protectors is that 
they are open on two sides, but when you insert them into a 
celbook, the static cling keeps your cel from sliding around.  
The Itoya books are acid-free or archival as they are called and
are marked as such.  Not sure about the paper inside, though. 
I wouldn't trust anything that isn't marked acid-free or archival.
If there's acid in the paper or the plastic, it will eventually
yellow your cel and in rare instances cause melt marks.  It'll 
also "suck" the moisture out of the paint causing it to crack.

Hope this helps.

Metaphor (i'll be broke for christmas, can't afford no cels... :,(  )