The best way to purchase a mattress is by visiting a few mattress stores and trying them out.
Ask about mattress construction and where the mattress is made.
Some mattress stores sell mattresses made in China. These types of mattresses should be avoided due to questionable material.
Poly foam which is in mattresses is made from oil. (You do not want oils such as used motor oil in your mattress).
If the retailer does not readily answer questions or does not know, then try another mattress store.