[freebeard]

What you see in the picture was cut from a scrap/remnant/cutoff left after MultiCraft sold less than a full sheet. Try sign shops. I've gotten larger pieces at BRING, the local recycler; typically an outside corner when someone cuts a circle out of a square.

Polymetal or equivalent claims rigidity equal to 5/8" plywood, but it's 3 [or 6mm, or with thicker skins] and waterproof. Apparently its used for wings on dirt track race cars.