Hi Bradbaker1989.
Welcome to our Mattress Forum! 
I am not sure how old your existing innerspring mattress is, but as it seems that it started to soften and break down then it’s much less likely that a topper will be an effective solution because you can’t compensate for soft spots or dips in the mattress by adding toppers because they will just bend into the softer areas underneath them and any effect they have will tend to be partial or temporary at best. if your mattress is not old and is covered by warranty, your best option is to get it replaced as a warranty claim for excess body impression. If it is old, or cannot be replaced under warranty, then attempting to make an older, excessively worn with body impressions mattress last a good while longer by replacing your tempur topper with another topper (whether HD, HR, Aerus or any other material) is not something I would advice.
Even though you love your tempur topper, I would not venture to build a mattress “around” it because a topper’s primary function is to contribute to the overall comfort and the most important priority is your sleeping posture and alignment that comes primarily from the support layers within your mattress. If you sleep with your spine or joints out of alignment then you may not feel that something is wrong until you have slept that way for some time and your body begins to produce “symptoms” once you have slept out of alignment for long enough. In looking for the best mattress/topper combination I would also keep in mind that all the layers in a mattress interact together and sometimes even seemingly small changes can change the feel and performance of a mattress quite significantly.
The only way to know for certain whether a specific mattress/topper combination is a good “match” for both of you in terms of comfort, firmness, and PPP is based on your own careful testing or personal experience on the combination. If you can’t test the combination in person then there will always be always some risk and uncertainty involved shopping for a mattress that will pair well your topper because the specifics of the mattress itself along with your own body type, sleeping position, and preferences can affect which specific mattress would be a suitable choice to work with your specific topper.
This being said you have the option of creating your own DIY with either a poly, or latex core layer, or with a new “regular” innerspring mattress. (You can probably find a new regular innerspring mattress at a pretty good price that you can negotiate at a local retailer) When shopping for a new mattress or DIY componentry I would especially make sure you read the durability guidelines here and post #4 here ~ about the factors that can affect durability and the useful life of a mattress.
While a2.8 lb poly base would be a good and durable material … It’s not possible for me to assess how a 35 ILD in combination with your existing topper or combinations of materials or components will work for your specific needs because the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP (Posture and alignment, Pressure relief, and your own Personal preferences) or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
We have decided to discontinue the provision of listings of potential retailers in various geographic regions (unless they are already approved site members), because of the difficulty in maintaining such lists in a retail landscape that is constantly changing, and most importantly the confusion it was creating with the consumer members who incorrectly assumed that these businesses had indeed gone through the strict qualification process and were approved as members of The Mattress Underground. Such an assumption is unfair to both the consumers seeking assistance, as well as the very businesses and manufacturers who meet the criteria to become Trusted Members of The Mattress Underground.
You can perform a forum search and you can type in the key term any city or area you would be closer to and consider driving such as Lexington or any other area and see what other businesses have been discussed in that region which may be helpful to you. This being said there In the last section of Post #3 there are listed some outlets in Lexington, KY that you might find useful in your search.
I would avoid placing your topper on a cheap firm memory foam mattress, as it will likely wear out or break down because foam softening and the breakdown or permanent compression of the materials or components in the mattress which will have different effects on different people. Any specific estimates for how long a particular person might sleep well may be a “rough guess” at best, but what can be said with certainty is that higher quality materials take much longer to soften or change and will maintain their comfort and support and last longer than lower quality materials. Generally speaking latex is the most durable of the different types of foam materials, higher density memory foams and polyfoams will last longer and maintain their properties longer than lower density foams of the same type, and that the layers both above and below any specific material or component in a mattress will affect the durability of that material as well.
Phoenix