Hi johnpg,
You can read a little more about the challenges involved in trying to match one mattress to another in post #9 here.
In most cases, a manufacturer would be the best source of information about other mattresses that would “approximate” one of their mattresse because they have more information about their designs than anyone.
Other than this if you try to “match” the density and thickness of the top memory foam layer and the density, approximate thickness, and ILD of the base layer and a similar cover it would often be a good approximation but keep in mind that even memory foams that have the same density can vary considerably in their specific properties and “feel” (see post #9 here and post #8 here). It can be difficult to find out all of the information you need to make close approximations wiothout the help of the manufacturer themselves.
Rocky Mountain Mattress certainly has good quality and value but a more detailed conversation on the phone with them would be the best way to have an idea about the suitability of their mattresses for any particular person or the guidance you would need about which mattresses may “approximate” them.
Phoenix