If you live near a hobby lobby they have foam mats usually 60x24 in different thicknesses. I usually just buy to and cut them evenly for wider tanks.
www.hobbylobby.com
Cosplay High-Density EVA Foam | Hobby Lobby | 1920461
Get the right material for the job! Cosplay High-Density EVA Foam has a superior, dense tear-resistant construction with a smooth surface. Shape and mold the material by applying heat. This material is great for cosplay costumes, crafts, and more!