When plexiglass/acrylic is poured into the glass mold, the standard mold is 4´x 8´
3´x 6´was already cut from the original size.
If you can´t locate a plexiglass manufacturer nearby, look for your nearest luminous signs (I guess that´s the english name for the large signs illuminated from the inside by neon tubes) manufacturer. These companies buy 4´x 8´plexiglass by the stack and then paint them on the inside. They should be able to sell you a single sheet, either white or transparent.