Hidgara007,
I thought from your comments that you lived in San Antonio but I should have paid more attention to the thread title and as you already know there doesn’t seem to be any good foam sources in Corpus Christi. I did call Burgess Fabrics and Upholstery but their selection is very limited, only in furniture sizes (they would have to order mattress sizes) … and they didn’t even know the foam density of what they sell.
If you do decide to go in a complete DIY direction then I would read post #15 here. You will need some good knowledge and experience with local testing to know the types of layering that you do best with or alternatively have a very wide range of the type of mattress that you are comfortable with (someone who can “sleep on anything”. There isn’t a “formula” that can take all the many variables of form types and combinations, body type, sleeping styles, and individual preferences and perceptions into account and predict with any degree of certainty how well a certain layering will work for any specific person. One person’s “too soft” is another person’s “too firm”. There is some more detailed generic information in the mattresses section of the site which can give you some insights into the effects of different foam layers, firmness layers, and thicknesses but knowing a specific design that is best for you will depend on your personal knowledge, experience, and testing or the knowledge and experience of a mattress manufacturer or retailer you are working with online or “at a distance”. It can often be less costly and more effective to work with a manufacturer that offers the materials you want with exchangeable layers to reduce the cost of mistakes (again unless you can “sleep on anything”) and that also provides a suitable cover for the mattress.
This would depend on the “type” of softness you mean (post #15 here talks about surface softness, pressure relief softness, or support softness) but softening a middle transition layer would tend to both increase pressure relief and reduce “support” and whether this was a good idea would depend entirely on how your body type and sleeping positions interacted with the specific combinations of materials and components in your mattress. The memory foam layer will be the biggest part of the “pressure relief softness” of your mattress which.
Phoenix