Actually, I didn't realize what you meant by old 
pizza boxes. I was beginning to wonder how much 
pizza you ate. ^_^;;;;

If you're running out of cardboard, you might be 
able to get more from a major toy store or even 
better cardboard from a liquor store. Both those 
types of businesses carry stock that has to be 
removed from (usually good-sized) cardboard boxes 
before it can be displayed. The boxes from the 
liquor store are usually better/thicker since 
they're designed to protect glass bottles.

If you're looking for something tough enough to 
ship a cel in, but which you don't have to cut to 
size. . . . . I can't think of anything. It would 
be possible to buy foam core by the sheet and get 
the store to cut it into smaller pieces. But the 
cheapest place I know to get it charges about $10 
a sheet for foam core - an expensive alternative.

Many Sharp Smiles,
--Drac