mattress topper to make a soft mattress hard!!

can anyone point me in the right dircetion to purchase a good quality mattress topper for a fixed bed.

We have the new bailey Orion 430-4 and although the bed is comfy, both of us have had problems with our backs for the last couple of weekends after sleeping on the otherwise very comfy mattress, at home we have a firm mattress and wonder if this matress is too soft for us and wondered if there is such as thing as a firm mattress topper rather than a soft one?