Hi scubamom,
Cardboard is fairly common in lower budget mattress foundations.
There is more about the types of foundations that are most suitable for different types of mattresses and some good sources as well in the foundation post here. In general the most suitable foundation for a heavy all latex mattress is a solid non flexing slatted foundation that has slats that are less than 3" apart so that there is some ventilation under the mattress.
I would make sure you have confirmed that the foundation is the source of your problem though before replacing the foundation.
Phoenix