Coroplast Plastic cardboard 0.157-in T x 24-in W x 36-in L Black Corrugated Plastic Sheet in the Polycarbonate & Acrylic Sheets department at Lowes.com
White 4mm Corrugated Plastic Sheet (24" x 18")
12x18 4mm Corrugated Plastic Sheets 100 Pack White Waterproof Lightweight, Blank Boards Double Sided for Lawn Signs, Garage Sales and Real State. Various Sizes, Colors by WholesaleArtsFrames-com - Walmart.com