Hi dormire,
Just in case you haven’t read it yet … I would start your research with the tutorial post here which has all the basic information, steps, and guidelines that you will need to make the best possible choice.
I have no idea about how to choose a mattress for a Shih Tzu but hopefully what is “best” for you will also be best for him as well 
Some of the better options and possibilities I’m aware of in the Atlanta area are listed in post #2 here
Phoenix