Went down to the local art store (well, it's just 
one block from my apartment) to find some 18-
plus" art portfolios.  Non-archival portfolios 
ran from $20-30, but the nicer archival 
portfolios ran $70-100.  I picked one up for 
$83.  It's called the "Start 2" by Prat/Europa.  
This particular one was 18" x 24", but they also 
had one that was 17" x 22" as well as smaller 
sizes.  It basically has everything you'd want in 
a portfolio (zipper, handle, etc. . .) and comes 
with 10 archival pages with acid-free black 
inserts.

I also bought another 11" x 14" Itoya art 
portfolio for $10.  I almost wish Itoya made 
larger portfolios than 14" x 17", but my extra 
large cels are kinda expensive so the extra costs 
with regards to protection are worth it.