Hi jackback,
It probably is their firmest ILD with the softest topper they have available but it’s not surprising at all that it feels very different to you because they are very different materials and combinations and different combinations of materials often don’t “translate” into another material or combination. I would focus on your own personal testing when you are considering any material that you haven’t tried in person because “theory at a distance” may not be particularly accurate because firmness and softness and how any material “feels” is very subjective and individual and in the end only your own experience can really know how any combination will feel with any certainty.
It sounds to me as well like your topper may have fibers inside (polyester or wool etc) rather than polyfoam as well. Without knowing what is inside it though the only way to get any sense of how a different topper may feel even on the same base mattress (your futon) would be your own personal experience and on a different mattress completely the variables become even more difficult to predict except in the most general sense (which is what they did because they at least recognized that you had some type of soft material over a very firm sleeping system).
I would agree with this that no latex will have a similar “feel” to a cotton futon even if they were both a similar firmness because of the natural resilience of latex. If you were to buy a cotton futon even online though it would likely be quite similar to your futon when it was new.
I haven’t searched the area for futons but the better options and possibilities I’m aware of in the Ann Arbor area are included in post #3 here and at least one of them http://www.dragonslairfuton.com/ appears to carry futons.
A quick google search of “futon, Ann Arbor, MI” turned up a couple of other futon possibilities that may be worth calling and there may be more further into the search results.
http://www.mattressandfutonshoppe.com/ann-arbor-mattress-stores?chapter=10
Phoenix