At the top of the forum there are links that say "Find coroplast" and "Find cubes" and I think you can type in your area and it will help you find the items. Cubes are generally really easy to find; I got mine at Fred Meyer, or you can go to Target, Walmart and Bed Bath and Beyond. Just make sure you're getting the right size. Each grid should be 14 inches, some stores sell 15 inch cubes but the squares on these are too large and the guinea pigs could get stuck or hurt themselves.
As far as coroplast; find a sign shop in your area. Sign shops always carry it and usually at a reasonable price, depending on where you are. I paid $25 for one sheet but that is not the usual price. We live in the middle of nowhere though.