Cels Forum

They make cel books slightly larger than the one you have,
you can check random cel pages that carry cel books and look
for them (I think Usagi's House and Noriko's Anime Harbor have
them). If the cels you have are still too large, and from my 
experiences I've seen some larger cels O_o...you might think 
about buying an art portfolio to store them in. I'm currently
shopping for a portfolio to hold my artwork at school, and they
come very large. Also, some have elastic bands on the inside 
rather than sleeves so you can lay your cels down in that and 
secure them with the elastic. Portfolios run a little high though.
Remember that cel books are basically just art portfolios so
you could try looking in art shops for portfolios that hold 
larger artwork.

valerie