Hi pastacats,
Post #21 here (also linked in the tutorial post) includes all the members of the site that sell online. All of them are recommended manufacturers or retailers. Many of them make “choose your own layer” latex mattresses with a wide range of different designs and options.
Each of them have more details about their mattresses on their websites and of course I would also make sure to talk with the ones that are of interest to you. I would also make sure you are aware of any shipping costs involved in exchanging or returning layers. In many cases topper purchases or individual layers that are purchased separately are not returnable at all.
Probably yes (at least based on averages) but it would depend on the specifics of the layers and your own testing. Post #2 here has more about the best ways to choose a mattress that is the best “match” for you in terms of PPP.
Post #6 here has links to all the forum lists that are closest to you and includes the better local options or possibilities that I’m aware of (they have some overlap but Magic Sleeper in Pottstown and the Organic Mattress Store in Hellertown would be among the better local options).
Phoenix