Mattresses come in many forms. You can take a spring mattress, but a more modern one with interchangeable sides for summer and winter. You can get a foam mattress. It is best for you to go to a shop in your town and decide. And then through the Internet to buy for example here

Thank you for your advice. I will definitely shop around and decide on my choice. And thanks for the link. Because I see they have a pretty good selection of bedroom products.