I bought some extra HDPE sheets this year and ended up with more than I needed for the project. Anyhow, if you are looking for larger 1/4 sheets of two-color HDPE you can find it here: https://www.amazon.com/ColorCore-Sheet-Black-Thickness-Length/dp/B00V83VYDA/ref=sr_1_1?crid=BA8MATSL6EZL&keywords=colorcore+plastic&qid=1577753300&sprefix=colorcore+%2Caps%2C139&sr=8-1
I am selling these through a managed listing, which is why it shows offers from colorcore and other venders. I should also mention that finding these on amazon was a huge PITA, but they are available, usually with free shipping.
White Red White
Red White Red
White Black White