Larger cities usually have stores that specialize in all natural product mattresses.
One good choice is to get a natural latex foam mattress, it has no petroleum chemicals in it, they do not off-gas. When you shop for them be sure that it is natural and sourced from trees, not a "synthetic" latex which is a misleading advertising lure because it is not real latex. Latex is a product from trees. Natural latex will outlast the chemical foam product mattress by a decade or more. They cost more than the petrochemical foams but you are getting good value because it is not going to compressed and stay in that flattened shape in only a couple of years.
They do come in different densities and softness. You can get them with all natural, chemical free matrtress coverings. These are a favorite of people who live on boats. The boaters get the type that have small holes that go through the whole surface.makes them a little cooler to sleep on, it also makes them lighter in weight but it does not compromise the support or the longevity. They do not hold moisture, they do not promote mold growth, it is naturally antimicrobial. It does not break down and it does not hold body impressions.
There are of course other choices in mattresses for people who have chemical sensitivity such as spring mattresses with wood padding and or a layer of latex foam.
As you are looking for a standard size mattress for home use you should not have too much difficulty in finding a store that stocks them. The best way to test them is to go to a store and lie down. Take your time, relax, you need to give your body time to settle in for a while. Do that little slow breathing, calmly relaxing your toes, then your fingers, then your legs and arms and especially your back and neck. If you don't let your muscles settle you will never know exactly how it will feel when you want to get to sleep. Wear shoes that are easy to slip off and on when you go mattress shopping. What you choose will be a choice given by your comfort and your wallet.