Try this site for oversized bags:

http://www.shimojima.co.jp/heikoinc/products/prod
ucts.htm

Click on OPP bags.  They're the same bags a lot 
of collectors in Japan use, except that you can 
now get them domestically from Hawaii.  Since 
I'm guessing you're in Japan, you should have an 
easier time finding the oversized Heiko bags.

As for storage, you'll just need to spend a lot 
of money getting a large portfolio from an art 
store.  My 18" x 24" portfolio cost about $80-
$90 or so from a local Utrecht store.
(utrechtart.com)

Framing is another possibility.  At least with 
hankens, line fading won't be such an issue.  So 
in that case, you can just get a cheap poster 
frame.  Place the cel in a cel bag and then put 
that in a poster frame.

As for my cels, I actually went out and got them 
professionally framed.  Just whatever floats 
your boat.  Later.