For those of us in the UK, I see Lidl have got their Chopping Boards on sale again. If, like others of the past, these are a cheap source for strong plastic sheets which can be very useful for items to be carved or as Test pieces. Give them a try before they sell out. Ernesto Chopping Boards - at Lidl UK Gray
Unfortunately it doesn't say how thick the boards are. If they are, at least, 6mm thick it's a great price for two of them.
Just checked last ones I bought and they are 8.5 thick Actually not sure now. The ones I have were bought individually, not for 2. So, not sure really. I'll have to see when I go there.
Mary, if you have TAP Plastics, they sell in 12 by 12 increments. I have found 10 by 14 pieces in their scap bins and it is sold by the pound. Not all stores may do that, but here I grab what I need and weigh it on their non- certified scale and then they knock a few pounds off to give me the benefit of the doubt. I have found many larger useful pieces for next to nothing in HDPE, acrylic, ABS, and polycarbonate. Sadly, I have never found any colored pieces in their bins.