Vegan mattress advice

Do you have a mattress suitable for me? I want it to feel firm and flat. I have been used to sleeping on a futon for a long time but this has now developed a hollow where I sleep. I like to sleep on my back a lot and sometimes feel pressure on my heels, so I want the surface to feel softer but level. I don't want to feel like I am sinking in. I am also looking for manmade fibres only as I am vegan. I weigh 9 stone, my husband 12 stone. Can you help please?