I get most of my large cardboard either from the local sears store or from a friends body shop. However, I have pretty much stopped using cardboard and use coroplast. Sure it's more expensive but it's weather resistant, hole close back up a bunch and you can use a piece for a long time. It's the same stuff most political or relestate for sale signs are made out of.
We have used several things and the Coloplast works the best of anything we have tried and is not expensive in the long run. We got ours from a sign shop that we use for $12 per 4X8 sheet.