Hi Holistically Yours,
Both manufacturing processes can use either synthetic rubber (SBR) or natural rubber(NR) or any combination or blend in their manufacturing. You can read more about the different types of latex in this article and in post #6 here along with post #6 here.
The best way to ensure you are getting what you want is to deal with reliable and reputable manufacturers and/or retailers that will tell you exactly what they use in their mattresses and the type and blend of any latex they use. There are only two primary manufacturers of Talalay (Latex international and Radium) but there are many good quality manufacturers of Dunlop latex and a reputable manufacturer will make sure that the materials they use has the best quality and value available to them and will tell you exactly what they are using. Some of the Dunlop manufacturers that you will often see includes Latex Green, CoCo Latex, Latexco, Arpico, Latex Systems Co, Thaitex, and New Zealand Foam Latex but there are many other good quality Dunlop manufacturers.
There are only two manufacturers of Dunlop in the US which are Sapsa (made by Sealy under license and who used to own them) which produces mostly synthetic Dunlop and Latexco which pours some of their softer Dunlop topper materials called Puralux in the US. None of the 6" 100% natural Dunlop cores you will see are poured in the US and most are from Asia.
Posts #2 and #4 here have the Connecticut and Connecticut/Massachusetts border (Springfield) choices.
The Boston list is in Post #2 here.
That should cover most of the region around you.
They could probably use either type of Talalay latex that you choose (they would have access to anything that Latex International makes) but I would call them for any specifics about what they have on their floor or what they could make with a special order. Anyone there can help you.
Hope this helps
Phoenix