Most mattress manufacturers have a so-called 'orthopaedic' range but there's no universal standard for these products. It would be better to seek out the advice of experts.
The charity, BackCare, (formerly known as the National Back Pain Association) have a booklet about choosing the most appropriate type of bed. At just 95p it could prove to be a very worthwhile investment: http://www.backcare.org.uk/catalog/detail.php?BID=20& ;ID=404
My wife suffers from a terrible bad back and we spent a fortune on an orthopaedic mattress, all nice and firm etc. She couldn't stand it and eventually we had to buy a feather bed to go on top of it where she reposes as if lost in a huge yorkshire pudding, ( apparently the worst thing for bad backs) but very comfy for her, so be careful what you buy:)