Beware of mattresses labelled 'orthopedic' - it is just an excuse for a very hard mattress and that might not be what you need.
Choosing the Best Mattress for Lower Back Pain
Find a mattress with back support. A good mattress should provide support for the natural curves and alignment of the spine. The right amount of back support also helps the patient avoid muscle soreness in the morning. While there is not much clinical data about mattresses, one study found that medium-firm mattresses usually provide more back pain relief than firm mattresses.