There are a few companies who sell portfolios 
with archival sleeves (& refills) 18" x 24" in 
size.  However, I have quite a few cels which are 
larger than that.  So, those cels are currently 
staying in a psuedo-case made out of cardboard 
underneath my bed.

You can try archival poster sleeves as well.  I 
believe those can be larger than 24" in size.