Sorry to hear about the sore back. When choosing a mattress, pay attention to your preferences – if you like softness or prefer firmness. Stores often provide trial periods where you can return the mattress if it doesn’t fit. It’s easier to choose through an online store at the link here.
If your back hurts, pay attention to the stiffness of the mattress. Maybe try models with orthopedic effect – they support your back well. And don’t forget to test the mattress in the store, lie down on it and feel how it supports your back