A lot of people go to Dollar Tree and buy their "boards" - cut them up to whatever size they need - they are, of course, $1 each for a very large piece. Also, you can check with your local sign maker - the ones that make the signs for political signs - the ones that the politicians put all over town during an election. They can sell you big sheets of it and possibly even cut it for you - don't know how expensive it is though. Doubt if you would want to use any "leftover" signs as they would have paint on them and it might come off on your fabric.