I have found a place to get egg crate at a reasonable price.
Also a thicker sheet of egg crate 1/2",but only in white.
The name of the place,prices and dimensions are below.

eplastics.com
Ridout Plastics Co. Inc.
5535 Ruffin Rd
San Diego, CA 92123
Toll Free 1-800-474-3688
ph 858.560.1551
fx 858.560.1941
Hours of Operation: M-F, 7am-5pm PST

.375"X2'X4' White Egg Crate.$13.61 Per Sheet.
.500"X2'X4' White Egg Crate.$22.73 Per Sheet.
.375"X2'X4' Black Egg Crate.$13.50 Per Sheet.