Hi mattresslook,
The first place I would start your research is the tutorial post here which has all the basic information, steps, and guidelines you will need to make the best quality/value choices based on any criteria that are important to you … and perhaps more importantly it can help you know how and why to avoid the worst ones (including any mattress that uses less than 4 lb memory foam which would be the case with many of the mattresses you looked at).
There are many different types of memory foam and even in the same density they can have a very wide range of properties and firmness levels (see post #9 here and post #8 here) so you won’t be able to tell the firmness of a mattress just by specs alone and I would make sure that you have a more detailed conversation with any online manufacturer you are considering so they can help “talk you through” your choices and help you make sure that any mattress you are considering meets the criteria that are most important to you. In very general terms though … mattresses with thinner layers of memory foam will tend to be firmer than thicker layers.
The tutorial post also includes a link to a list of the better online memory foam options I’m aware of as well. Many of these have designed some of their mattresses to be close approximations to the firmness of many of the Tempurpedic models so if you know which Tempurpedic mattress has the firmness level that you prefer then it would be easier to choose an online mattress that is similar.
If you let me know the city or zip code that you live I’d be happy to link you to the better options and possibilities I’m aware of as well because it’s possible that there may be some local options that meet your criteria that you could test in person.
I don’t keep a list of mattresses based on specific specifications or height so you will need to either check their websites or talk to them on the phone to find a mattress that is suitable but there should be quite a number of options that would be 9" or less that would be likely meet your criteria.
Phoenix