Hi Bulgingdisk.
Welcome to our Mattress Forum! 
It is quite fortunate for you that you did a bit of homework as the major brands such as Sealy/Stearns & Foster, Simmons, and Serta all tend to use lower quality and less durable materials in their mattresses than most of their smaller competitors that will tend to soften or break down prematurely relative to the price you pay which is why I would generally suggest avoiding all of them completely (along with the major retailers that focus on them as well) regardless of how they may feel in a showroom along with any mattress where you aren’t able to find out the type and quality/durability of the materials inside it (see the guidelines here along with post #3 here and post #12 here and post #404 here ).
While you usually won’t find an mattress that is “equivalent” to a “target” mattress you have tested locally made by a different manufacturer… in some cases the “art and science” of approximating a mattress has already been done to a large extent and an online manufacturer or retailer may have already “matched” their mattresses to a specific mattress. In other cases they may be aware that one of their mattresses is a close approximation to another mattress based on their own personal experience (even if it isn’t in the mattress description) so if the mattress it approximates is available locally then this would give you a chance to test a specific local mattress that was a reasonably close approximation to a mattress you are considering online. Attempting to find something that is exactly the same would be an exercise in futility and often a frustrating experience especially if you are trying to do that on your own. There is more information in post #9 here about the different ways that one mattress can “match” or “approximate” another one. This being said you could be on the right track by seeking guidance from an experienced manufacturer or retailer, who would be much more qualified to best approximate a mattress feel based on all the specs and the personal needs and preferences you may provide.
While the many mattress options on the market, the amount of information and misinformation can be overwhelming if you approach the shopping experience in a more “scientific” way by narrowing options based on quality/value, suitability and on your own personal value equation you can quickly realize what to look for and where to look. I would suggest to not be tempted to rush the process but to approach it systematically. You’ve already narrowed the “number of choices” based on the quality of the componentry within a mattress and although I’m not sure what you’ve read since you found us, I would suggest starting your research with the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly know how and why to avoid the worst ones.
Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort”, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
I hope this gives you a good start and once you have a chance to narrow down your options please let me or any of our Expert Members know and if you have more specific questions we’ll be happy to assist you.
Phoenix