Hi cbear08.
Welcome to our Mattress Forum! 
I moved your post and changed its title to something easier to find for those that are interested in in a similar subject.
The good news that you certainly got your money’s worth out of your mattress, but attempting to find something that is exactly the same is often a frustrating and futile exercise especially when trying to duplicate an old mattress for which you don’t know the componentry and specifications. Even if you somehow manage to find them you’ll still have to do a great deal of testing, research, and legwork. You can see more about the several ways that can be used to “approximate” or “match” a mattress to another one described in more detail in post #9 here
Every individual layer and component in a mattress (including the cover, FR barrier, any quilting material, and of course all foam layers) will affect the feel and response of every other layer and component both above and below it and the mattress “as a whole” so you would need to carefully asses the design and the specs of each mattress that uses exactly the same type of materials in order to determine if that the mattress in its entirety will be a good “match” for you in terms of “comfort”, firmness, and PPP(Posture and alignment, Pressure relief, and your Personal preferences) (see mattress firmness/comfort levels in post #2 here).
Most people have little idea about how difficult it may be to duplicate another mattress as it greatly depends on whether you are trying to duplicate just the “feel”, the design, the quality of materials, or all three. Post #2 here also has more about trying to duplicate another mattress. The limiting factors in success are the ability to get accurate information about the mattresses you had in the first place (so you know what materials you are duplicating) and your ability to find the exact same foam in the same thickness at various suppliers and your ability to also match any other design differences in the mattress (construction method, covers, quilting etc). Every difference between your mattress and what you are trying to duplicate can make a difference in how it feels and performs … sometimes in surprising ways for those that aren’t used to working with mattress materials, foams, and components.
The best you will hope for is to find something “in the range”, but realize that only your own careful personal testing or a detailed conversation with an experienced and knowledgeable manufacturer/retailer that has your best interests at heart will reveal whether or not the feel you are looking for is close to your needs and preferences.
Unfortunately …it would be almost impossible for anyone to figure out the exact specifications of the componentry within your old mattress going just by the description you provided of how the mattress feels to you because this is very subjective and only you can feel what you feel on this particular mattress and someone else may feel just the same on a totally different mattress that would be completely unsuitable to you. Although I do not recommend this in your situation, if you are trying to replicate this mattress or approximate its feel, perhaps you can look for a law tag that mentions the percentage by weight of any materials within the mattress and can specify what type of mattress this might be (Innerspring product, latex etc.) But at this point I think finding the exact specifications would mostly useless to you as within the 15 years that passed your body aged and changed some of its needs and also, of course, the mattress gradually changed before it broke down to the point that you are no longer sleeping well on this mattress. This is a long way to say that this would be very a complex situation with slim odds of success which introduces even more variables that you would have if you started from scratch. You would be probably better off by learning about getting quality componentry and trying to approximate the feel and comfort that works best for you.
I’m not sure what you’ve read since you found the site but just in case you haven’t read it yet … the first place to start your research is 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).
While again nobody can speak to how any specific mattress will “feel” for someone else or whether it will be a good “match” in terms of comfort, firmness, and PPP because this is too subjective and relative to different body types, sleeping positions, and individual preferences, sensitivities, and circumstances and you are the only one that can feel what you feel on a mattress … outside of PPP (which is the most important part of “value”), the next most important part of the value of a mattress purchase is durability which is all about how long you will sleep well on a mattress. This is the part of your research that you can’t see or “feel” and assessing the durability and useful life of a mattress depends on knowing the specifics of its construction and the type and quality of the materials inside it regardless of the name of the manufacturer on the label or how a mattress feels in a showroom or when it is relatively new so I would always make sure that you find out the information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
Once you’ve had the chance to read through the iformation provided, feel free to post back and I or any of our Expert Members of the site will be glad to assist you.
Phoenix