Hi richardb,
I just don’t know how to help you with the specificity of what you are looking for. You will need to call sleep Science to find out the details of their layers (if you don’t know them) and then you will need to source the latex layers to replace it. If you believe (which I don’t) that latex from Thailand is the only latex you would be happy with then you may need to call them in Thailand and find out who carries and sells their latex over here. The better latex sources I’m aware of are listed in post #4 here but if what you want isn’t available through them you would need to research further to find other sources.
I’m just not sure how else I can help you. If I was in your shoes I wouldn’t be going in the direction you are going in the first place (or in the direction you have already been that didn’t work out) and would either buy something locally that I had tested and worked well or would choose a manufacturer and build a latex mattress that had the best odds of being suitable and had exchange possibilities if you made a mistake.
Phoenix