You can find suitable portfolios or presentation
cases for housing your cels in many places.  You
can try your local art stores for them (Itoya and
Prat are some brands of these portfolios).  Also,
you can get them from online cel dealers
(Noriko's, Usagi's House, Nichibei, Asylum) and
online artshops (Dick Blick, Utrecht Art).

I tend to use the 11x14 inch Itoya portfolios, but
I have some other sizes and manufacturers too.

Hope this helps!
Keys