Hi averyb,
The better options or possibilities for mattresses I’m aware of in the Los Angeles area are listed in post #2 here but I don’t know which of them carry toppers off the top of my head so you will need to check their websites or call them to find out which of them carry the types of toppers you are interested in testing.
If the top few inches of a mattress uses the same materials that would be in a topper it would also give you a general sense of what the material feels like as well.
I would also keep in mind that the softness/firmness of a mattress under a topper can have a significant effect on how a topper feels as well so the same topper can feel different on different mattresses.
Phoenix