Hi,
I think a cel book is probably the best and safest
way to store your cels.  I use the Itoya Art
Portfolios, they are archival / acid free.

Here is a website online that has some for sale:
http://www.darkharbor.com/trades/html/celbooks.html

You can also most likely find them at your local
art supply store.  I know I have found them at
Hobby Lobby in several sizes.