http://www.utrechtart.com/dsp_view_product.cfm?
classID=2210&subclassID=221011&brandname=Prat%
2FEuropa%20Inc%2E&Item=24615

That's for an 18" x 24" portfolio/case and comes 
with 10 pages (20 cels).  You can always add more 
pages if you want.  I keep all my > 17" pan cels 
in there.  Depending on the cel, the pages may 
also have enough room to show the cel and gengas 
side by side.

They also carry a 17" x 22" portfolio.