Hi rssuttle,
The first place to start your research is post #1 here which has all the basic information, steps, and guidelines along with the most helpful forum posts that can help you make the best possible choices.
The best way to choose a mattress is either your own personal testing using the testing guidelines in the post I linked or if you are looking at an online purchase then more detailed discussions with the retailer or manufacturer who will know more about their mattresses and which would be most suitable “on average” for different body types and sleeping positions than anyone else (see mattress firmness/comfort levels in post #2 here). In some cases they can suggest mattresses that are available locally that are good “approximations” for some of their mattresses that you can test that can give you a good reference point you can use as a guideline for your choice.
Phoenix