Hi emm0119,
In cases like this if you don’t have any personal experience testing specific materials as a reference point and you don’t feel comfortable with the guidelines I linked then you would need to keep your expectations realistic and would be dependent on the results of more detailed conversations with the retailers or manufacturers you talk with (that I linked earlier) about the specific toppers they sell. If they are experienced and knowledgeable and you provide them with as much information as possible about your experiences, preferences, and your mattress then they may be able to give you some insights and “educated guesses” about which of their toppers may be most suitable based on “averages” of other customers that who may be similar to you and have similar circumstances.
The other way that you can lower your risk is with a good return policy so if you make a mistake then you would at least be able to return the topper and try again at little to no cost.
Post #2 here has some links to some of the theory as it relates to body type, sleeping style and other factors but these are only generic concepts and there are too many variables, unknowns, and individual preferences to make specific choices based on “theory at a distance”. You would also need to know the specifics of the layers in the mattress you have. The guidelines are a way to choose based on your actual experience based on whether you need “just a touch to a little”, or “a little to a fair bit”, or a “fair bit to a lot” in terms of pressure relief. This way you can use your best judgement because only you can feel what you are feeling on your mattress.
I can also tell you that outside of lying on the topper on your own mattress that samples won’t provide you with any sense of how a topper may feel and perform.
I’m certainly happy with helping with how to choose (your own personal testing first if that’s possible, connecting with more knowledgeable and experienced retailers and manufacturers along with the guidelines second, and making sure that if you don’t have a reasonable level of confidence after these that you only buy from somewhere that has a good return policy third) but what to choose is always up to your personal value equation and risk tolerance.
The most common big box stores are Costco, WalMart, and Sams Club but some of the retailers on Amazon and some on the list I linked also have return policies. I don’t have a specific list of the return policies of each retailer or manufacturer so you would need to talk to them or check their site.
Phoenix