I've paid 25eur (30$?) for precut acrilyc sheets (3mm and 5mm clear transparent) and around 17eur (20$?) for precut plywood. Both was bought in local acrylic shop (they are making signs for companies) and wood store (they are mostly selling furniture and furniture pieces). The thickness of the panels can vary 1/1.5mm up/down, so can be bought in local dimensions.
Aluminium extrusion can also be bought from Aliexpress or Ebay if you're willing to wait for it. I believe there was a guy showing the receipt last time for half that price. Carbon decals are really cheap... something like 10$ or so. I have some links for USA sources that people reported back, but not on acrylics and plywood as everybody is buying that locally.