@SteamLouis-- Also search for "orthopedic" or "theurapeutic" mattress. Although different names may be used, these are all basically different types of posture mattresses because they support the spine.
If you suffer from a specific spine condition, then ask your doctor about the firmness level. Your doctor may even suggest an exact type of posture mattress to use.
If you don't have any spinal conditions and just want a mattress with more support, I suggest getting one with medium firmness. Many years ago, doctors used to recommend very firm beds to their patients but this no longer applies. Now doctors feel that a medium firm bed is the best for the spine. So a mattress should not be too firm or too soft.