I know the largest that Home Depot carries is 4'x4', I think. One of the brand names for the corrugated plastic is Coroplast. It is available in sheets of 4'x8', like plywood, if you can find a dealer. Sign shops use it and if you can convince them you are using it for chickens and not to compete with them, they may sell you some of the big sheets. Otherwise, just Google Coroplast or corrugated plastic and see if you can find a supplier close by. It comes in different thicknesses, too, at a supplier if you want something sturdier.
ETA: Just looked again at Thespoiledchicken's brooder. Looks like your Home Depot carries the 4'x8' sheets. I wish mine did.