Try Here:
/www.harborsales.net
best to call on the telephone as the ordering process on their web page is not very intuitive and it grossly overestimates shipping cost. The people there are well aware of the use of coroplast for constructing RC planes.
I buy 4'x8' sheets and have Harbor Sales cut them to 2'x4' size to reduce shipping cost.
Live long, and prosper
Jerry L. Gubka