These are the .040 White Styrene Super Sheets from Midwest Products.

FEATURES: Solid white in color
Flexible...bends and forms easily
Compatible with many plastic adhesives
Compatible with most paints, including Testors, Humbrol Enamel,
Floquil Model Railroad Paint (solvent based)
Thousands of uses including architectural models, school projects,
fine arts, hobbies, do-it-yourself projects, and more

INCLUDES: Four sheets

SPECS: Thickness: .040" (1.00mm)
Size: 7.6" x 11" (194mm x 279mm)

COMMENTS: Some acrylic paints will rub off, so test before using them on your
project.

Midwest

Part MID70104