I’m sure there are other mattress dealers that do this, but I happened to find it on the SleepEZ site. There’s a component ordering page
https://www.sleepez.com/mattresscomponents.htm
and although they’re described as for building or enhancing a mattress, I almost see them as a way to prolong the mattrress’s useful life. For example, my soft latex top layer may wear out before the firmer layers underneath. If so, I can just replace that top layer anytime, even years after I bought the bed. A fraction of the cost.