Hi JohnnyO,
I think the “best” first step you can take would be to find the better manufacturers or sleep shops that are local to you. If you let me know your city or zip I’d be happy to let you know about any I’m aware of.
It would be great if one of the members was in the “list” close to you but there are also many local manufacturers and better sleep shops that have good quality/value mattresses that are not members of this site … yet 
In general … you will find that the OMF Serenity is already among the better quality and value memory foam mattresses of this type in an “apples to apples” comparison with other mattresses that use similar quality materials. The layering information about the Serenity is in post #2 here.
Even with knowing the layering information though … there are so many different types of memory foam with many different properties that the density information and thickness of the layers may not “translate” into other types of memory foam mattresses that use similar layering. In other words they may look similar on paper but not in real life. You can see more about the many different properties of different types and formulations of memory foam in post #9 here and in post #8 here.
There are really only 3 ways to “match” one mattress to another and they are listed in post #2 here.
Because of the many different types of memory foam made by different manufacturers and the differences in the layers used by different manufacturers … matching the Serenity based on matching the exact same specs may be difficult to impossible. A mattress that is somewhat similar may also be “somewhat similar” in its feel and performance but it may also be quite different unless all the layers are exactly the same.
The second way would be with side by side testing in “real time” (or at least with little time between them) which may also be difficult because OMF doesn’t carry other memory foam mattresses to compare it with and if there is too long a time between testing the Serenity and other mattresses the comparisons may not be that accurate (our subjective memory for the “feel” of a mattress isn’t very accurate over time). If there are other local possibilities though and you focus first on lying on the Serenity to “refresh” your memory and then go directly to some other pre-arranged options that you wanted to compare you would probably have a reasonably accurate comparison between them. A mattress that has similar materials and layering may be outside your budget but you may find a mattress that uses different layering with less costly memory foams (say without the 7 lb memory foam or equivalent) that feels similar and has a lower price.
The third way would be if a manufacturer designed and tested a mattress to feel very similar to the Serenity but unfortunately it isn’t a popular enough mattress to justify a manufacturer trying to copy it.
A mattress that is popular enough to “copy” though would be the Tempurpedic line and finding a mattress that is quite comparable to one of these may be much easier because many manufacturers use them as a comparison. In some cases these comparisons are for marketing purposes only and designed to make their own mattresses look better than they are but there are also better quality manufacturers that can provide meaningful comparisons of their mattresses to the Tempurpedic line which are much more accurate. This is more common with online manufacturers who often use the Tempurpedic line as a reference point.
Because of this … it may be worthwhile to test various Tempurpedic mattresses … especially the Contour line which also uses higher density 7 lb memory foam … and see which one comes closest to the feel and performance of the Serenity. Then you would have a baseline that would be a much more valuable reference point to compare to other memory foam mattresses.
The Serenity would be most similar to the Rhapsody that uses similar density and layer of memory foam except the Rhapsody doesn’t have the reticulated foam in between the layers and the firmness of the base layers may be different and you would need to confirm that it felt similar to you. You may also find that another one of the less expensive Tempurpedic mattresses has a similar feel in which case it would also have a less expensive counterpart with other manufacturers.
If the Rhapsody feels similar … then you would be looking for a mattress that was designed and tested to have a similar feel and performance to the Rhapsody (or any of their mattresses that you felt was similar).
Outside of local choices where you can feel the similarities in your own personal experience some of the better online memory foam choices (some members and some non members) are listed in post #12 here. You will find though that the mattresses that use similar quality materials to the Serenity or Rhapsody are probably more than your budget. In this case the most effective way to choose (with an online manufacturer where you can’t go by feel or local testing) would be with a conversation with the manufacturer where you could ask the questions that are important to you. I wouldn’t try to match a mattress based on the specs on a website … even if they are listed … because they may not ranslate into similar performance and feel. Conversations with the manufacturers or retailers on the list are the best way to go.
So your options are local manufacturers or retailers where you can go by feel or alternatively working with the online choices listed in the linked post who can match their models to the Tempurpedic that you feel comes the closest to the Serenity. Of course Rocky Mountain is one of these that I would definitely include as one of your phone calls. While I like the people at Bed in a box, they use much lower quality memory foam that would not be similar to the Serenity (or even most of the other Tempurpedic mattresses) in terms of feel, durability, or quality.
Hope this helps.
Phoenix