they're called bunky boards around here too. I never heard them used in daybeds, but they are always used on bunk beds.
I guess plywood might be an OK substitute, but not the really skinny kind, it would have to be the thick kind so it wouldn't bend or break with use.
I know I just couldn't skimp on something that my baby is going to be sleeping on so high up (top bunk). I think it was only about 60-80 dollars for the bunky board. If it was just for a daybed, I guess it wouldn't be as important.