I've been using a Prat Start 2 Presentation Case
(not Portfolio Case).  They go up to 24 x 18" and
come with pages.  You can also buy refill pages (I
have the pages mine came with plus a refill). 
There's one pocket were a store one that's too big
for that too.  They zip close and are easily
leaned up against a wall pretty vertically.  You
can check them out at many online art supply stores.

Keys