Hi ewlang,
Some of the better options and possibilities I’m aware of in the Philadelphia region are listed in post #4 here.
There are generally too many variables in mattress design and the types of components and foams inside it to be able to objectively match one mattress to another based on specs unless a mattress has exactly the same design and components (which is very unlikely) so it’s not really possible to “match” one mattress to another except with your own personal side by side testing or unless an online manufacturer or retailer is very familiar with the mattress you are considering and has the knowledge and experience to make a direct comparison of one of their mattresses to the mattress you are considering based on their own and their customers’ experience (you can read more about “matching” one mattress to another in post #9 here).
With local testing it’s generally more effective to match each mattress against a common set of criteria rather than using a particular mattress as your “target” (see post #46 here). With an online purchase your more detailed conversations with a retailer or manufacturer or their direct comparisons with some of the major brand mattresses are the most effective way to decide which mattress is the best “match” for you. I don’t know of any online manufacturer that compares their mattresses to any of the iSeries although some of them may be familiar enough with some of the iComfort mattresses that they may be able to make some comparisons with their mattresses.
Phoenix