Itoya's largest size is 14" x 17".  (Shouldn't 
cost more than $20 max.)  Above that, people seem 
to run into problems.  I'm going to ask the local 
art store soon about better cost solutions for 
extremely large storage sizes.  (There are 
archival portfolios at on-line stores that go up 
to 18" x 24", but they cost about $100.  
Different brand than Itoya.)  If you want to 
spend more, there are better brands than Itoya, 
of course.  Itoya just seems to be the best bang 
for the buck.

As for cel bags for unusual cel sizes, just make 
your own from smaller cel bags.  Cut out 
squares/rectangles and tape them together (tape 
obviously on the outside of the bag; I use 1" 
clear tape) to make a bag.  I wish I could take a 
photo of my 14" x 22" cel just to show how many 
bags (well, polypropylene envelopes) I cut apart 
to make a custom bag for the cel.  Until I find a 
better cost solution for the extremely large 
archival portfolios, I just store them in custom 
made cases (made of cardboard from priority mail 
boxes; I wish I could take a photo of that too).  
Later.