Hi joyous,
The first place to start is post #1 here which will give you the information, steps, and guidelines you need to make the best possible choices.
A mattress is as good as the materials inside it and you can only make meaningful comparisons between mattresses if you know the details of all the layers. As you can see in post #3 here and in post #4 here … the Big Box stores would certainly not be my first choice for a mattress and they know very little about what they are selling or which of their mattresses are suitable for different body types and sleeping styles. Unless you have tested a mattress they sell … there is really no way for you to know whether a mattress you are considering from them is suitable for your needs and preferences (unless you test either these mattresses or something similar). If it isn’t … then no matter what you have paid it would have little value to you and you would need to return it and start all over again (and the big box store return policies are the only good thing about them. They are generally not the best value either.
The “feel” of a mattress is very subjective and can be very different for each person. It depends on the body type, sleeping style, preferences, and individual perceptions of each person. Unless a manufacturer or retailer has specific knowledge or how their mattresses compare to the iComfort (and most wouldn’t have any reason to compare their own mattresses to them) …the only way for you to know how closely one mattress feels to another would be for you to test them side by side in a store. You can read more about trying to match another mattress in post #2 here (and it’s usually not possible except with your own personal testing). Generally it’s best to avoid the major brands like the iComfort completely because they will only end up taking up a lot of your time only to find out the information you need to make an informed buying decision isn’t available anyway and that they are not particularly good quality or value. I would also test every mattress against a common set of standards instead of the “feel” of another mattress.
I would follow the steps in the first post I linked.
The foundation thread here will be helpful but in many cases you will be able to buy a foundation from the same place as your mattress.
Phoenix