Amazon.com: Brown Chipboard 100 Point Extra Thick 8.5 x 11 Inches.100 Caliper Heavy Cardboard 2.54 mm Thick
Amazon.com: Chipboard Sheets 8.5" x 11" - 100 Sheets of 22 Point Chip Board for Crafts - This Kraft Board is a Great Alternative to MDF Board and Cardboard Sheets
Amazon.co.jp: White House Double-Sided White Cardboard (Chip Ball), 12.2 x 8.7 inches (310 x 220 mm), Slightly Larger Than A4, Approx. 0.08 inches (2 mm) Thick, Set of 5, A4, No. 26 (1.92 mm) Thick) : Hobbies