Hi sj_stella,
There are too many unknowns, variables, and personal preferences involved for anyone to make specific mattress recommendations for someone else based on specs (either yours or a mattress) or “theory at a distance” (see mattress firmness/comfort levels in post #2 here). I also don’t keep a list of all the mattresses available in any specific area based on their specs because it would be impossible to keep up with and is constantly changing … but if you let me know your city or zip code I’d be happy to let you know of any of the better options or possibilities I’m aware of in your area where you may be able to find a mattress that meets your criteria.
I would also tend to avoid buying a firmer mattress and then adding a separate topper afterwards unless there is no other reasonable alternative or unless you have a chance to to do some careful and objective testing for PPP (Posture and alignment, Pressure relief, and Personal preferences) on the specific combination in person using the testing guidelines in the mattress shopping tutorial. Deciding on a topper that would be a good match for both you and the mattress (the specifics of the mattress can make a significant difference in which topper would work best for you) can be almost as challenging as buying a new mattress in the first place.
In most cases you will also find that it’s less costly to buy a mattress from a good manufacturer or retailer that doesn’t have any lower quality materials or “weak links” inside it (that you are paying for) in the base mattress and that already contains the layer or materials that you would otherwise be adding as a topper.
If you do decide to go ahead with the additional uncertainty of buying a mattress and a topper separately then I would suggest buying the mattress first and then adding a topper that you think would work well for you after you’ve tried it and based on the information in post #2 here and the topper guidelines it links to that can help you use your actual sleeping experience as a reference point and guideline for choosing the type, thickness, and firmness for a topper that would have the least possible risk and best chance of success.
If you choose a suitable topper and the mattress/topper combination is a good match for you in terms of PPP then it would also have the advantage of being able to replace just the topper if it softens or breaks down before the top foam layers in the mattress (the top layers of a sleeping system tend to be the ones that soften or break down first) or if your needs or preferences change over time. A topper can also help extend the useful life of the mattress underneath it.
Phoenix