Needed a black cakeboard and couldn't find one anywhere. So I bought a black plastic table cloth and cut to fit and covered my cakeboard. I used scotch tape and then cut a smaller board and glued to the back to cover up the tape. Worked great and now I can have any size black cakeboard I want.
Have you ever thought about covering your boards with sugar? I have started making mine match the cake and it really gives a more finished look.
In Australia we can buy self adhesive food-grade PVC - its great for black cake boards.