Hi Thea,
Soft or even “super soft” mattresses are widely available in the industry at reasonable prices but if you are only looking at online mattresses then they would be less common because they would only be appropriate for a small percentage of people and the return rate would likely be much too high. Mattresses that are suitable for a smaller percentage of people would be more common with local manufacturers or retailers.
These are called component mattresses and have individual layers with a zip cover and they are certainly available with 2 or 3 or 4 or even more individual layers either online or in some cases locally as well. If none of the component mattress suppliers you are considering have the specific layers available that you are interested in then you can also purchase specific layers and components and a suitable zip cover individually from suppliers that provide mattress components and make your own DIY mattress.
You can also add a softer topper to a mattress that is too firm for you as well.
If you are attracted to the idea of designing and building your own DIY mattress out of separate components and a separate cover then the first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel” that may be useful as well.
For those who decide to take on the challenge then I would either use the specs (if they are available) of a mattress that you have tested and confirmed is a good match for you in terms of comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) as a reference point or blueprint and try and “match” every layer and component in your reference mattress as closely as possible or use a “bottom up” approach (see post #2 here).
Phoenix