+1 sounds about right to meprobably rowmark, johnson plastics sells it in 12x24. They call this size a quarter sheets because a whole sheet is 24x48.
That color blue, solid core, is from their ADA Alternatives line, PN# 341-501 for 1/8"... any trophy/engraving weenie can laser cut the blanks for you.
Looks Like PVC to me too. Slap on some white vinyl and you, my friend, are in the sign biz!!!