Hi fire_water,
Some of the better sources for mattress components or toppers I’m aware of are linked in the topper guidelines (post #4 here to save you going back).
@ Nevermore
Your comments and insights are great and very accurate but there are a couple of things that would be worth clarifying.
The bottom two layers (3" of “firm” latex and 3" of “medium” latex) would be a fairly typical “progressive” construction that is used in many mattresses. The bottom 3" layer would be a firm “support” layer (not a comfort layer) and the middle 3" would be a “transition” layer (which helps with both support and comfort and the “transition” between the support and comfort layers) and then the top few inches on top of this (either separate layers inside a common cover or toppers inside their own covers) would be the “comfort layers”. If the bottom two layers are still in good condition (which is likely) then all that would be needed is a few inches of softer materials on top. The key is that the “comfort layers” are suitable for a particular body type, sleeping positions, and personal preferences in terms of the type of material, their thickness, and their firmness so that in combination with the deeper layers they are a good match in terms of PPP.
The rest of your comments are “right on the money” and very helpful ad insightful IMO 
Phoenix