Hi mrmedoes,
The first place I would start (if you haven’t already) is post #1 here which has the basic information, steps, and guidelines that can help you make the best possible choices … and avoid most of the worst ones (such as major manufacturers like Serta and Tempurpedic who tend to either use lower quality materials, don’t disclose the quality of materials at all, or in some cases use better quality materials but have poor value compared to many smaller independent manufacturers).
You can read a little more about trying to match another mattress in post #2 here. Many online memory foam manufacturers will have a good idea about how their mattresses compare to the Tempurpedic line because they are so widespread and are often used as a comparison or frame of reference so people can test a similar mattress locally but comparing other memory foam manufacturers would generally require side by side comparisons (which of course isn’t possible with an online purchase) unless a particular manufacturer happened to be familiar with the one you wanted to compare.
The list of some of the better online sources for memory foam mattresses I’m aware of is in post #12 here and it would be well worth talking to them about your criteria and preferences. They can give you some good insights into which of their mattresses compare to the Tempurpedic mattress you tested in either “feel”, quality of materials, or in design (or in some cases all three) and some may even have some insights into how the iComfort compares to their mattresses as well if they are familiar with it and have made specific comparisons.
I would always start with some of the better local options and compare them to the online options you have available and this will give you a better frame of reference and some good comparisons based on your personal value equation so you can make the best possible final choices.
Phoenix