Hi Better,
Have you searched for ‘mattress topper’ instead of ‘mattress pad’? I find the word topper is more commonly used for what I think you’re looking for. There is definitely no shortage of mattress toppers out there 
several of the site members sell excellent toppers.
Convoluted poly foam is about the cheapest material you can buy, and rarely used in higher quality toppers. It doesn’t really surprise me that everything you see looks like it ‘sucks’.
There’s a good post, the URL of which I always forget of some toppers. Until then, one site that has a variety to look at from several manufacturers is
Also, sites like mattresses.net, SleepEZ, sleeplikeabear, all sell latex slabs and covers for them which are excellent toppers also.
Most common higher quality materials are latex, wool, cotton.
Also, make sure you’ve read the tutorial post on selecting a mattress. I know that you’re only inquiring about a topper, but the requirements are essentially the same, and the problem you’re trying to solve is one of the 'P’s in PPP - I.e. Pressure relief (read the tutorial, which will explain). The tutorial is here:
Essentially, the goal is to find a mattress and topper combination which provides suitable Posture and alignment and also suitable pressure relief. The good news is softening a too firm mattress is generally easier than firming up a too soft mattress.