Ok I'll let you in on my secret.
I go to Costco and just grab all of the cardboard that they have separating the layers on the toilet paper/paper towel pallets. All they do is recycle it so they don't mind if I walk out with 10-15 sheets of it for FREE. I do ask though just in case the security nazi thinks he's saving the day by tackling me for stealing FREE cardboard. Its worked out good so far. Not too thin and easy to manipulate into patterns and shapes.