They make "clear file" books in Japan at least up 
to B3 poster size (I have one of these that I use 
for my oversized cels), and maybe even A2 poster 
or larger.  Most regular size cels fit into what 
are called "A4L" or "magazine" files.  The next 
standard size up is B4, which fits most regular 
background and slightly oversize cels.  You could 
probably get away with an A3 file, which is the 
next standard size up from B4.  Just hope this 
helps you to know what to look for!


Jennifer