Hi JW3,
You really have a choice of two directions here. One would be replacing the sofabed mattress itself with a higher quality mattress and the other would be adding a topper. Both would isolate the feel of the metal springs and bars in the sofabed and be more comfortable than the typical sofabed mattress which is usually made of fairly low quality polyfoam (although they do vary in the grade and “feel” of the polyfoam they use).
If you decide to replace the mattress then a few of the better sources I’m aware of that make or sell sofabed, truck mattresses, or rv mattresses include …
https://www.sleepez.com/ A member of this site.
https://www.mattresses.net/ A member of this site.
www.mygreenmattress.com A member of this site
http://www.mattressinsider.com/ or http://www.sofabedmattress.com/
If you have somewhere to store it when it’s not in use and decide to go in the direction of a topper then post #4 here includes some of the better topper sources I’m aware of.
Even a 2" topper can make a considerable difference in the comfort of a sofabed.
Phoenix