Hi saramom,
There is more about bunkie boards (a foundation that is only about 2" high) in post #4 here. Regardless of which one you choose though I would make sure that the bunkie board you purchase matches Gardner’s warranty requirements.
I’m surprised that Gardner doesn’t have bunkie boards but if they don’t then many of the other local manufacturers or retail stores would probably carry them.
I would also be aware that if you tested your mattress on a flexing box spring rather than a foundation that a bunkie board with a solid non flexing surface can change how it feels
Phoenix