back SLEEPERS will have the largest selection
Generally back sleepers will choose something medium to medium firm. But some soft mattresses also work great for back sleepers. It really comes down to the quality of the mattress you buy. If its a quality mattress with either a great foam core or great coil system even a soft can perform well for a back sleeper.
I think medium firm is best because it will have a little cushioning on top. That cushioning will contour to your lower back better than a firm mattress. This will ensure you have excellent lumbar support.