Extra thick mattress topper (Green Park RG2)

Hello everyone, I have a big ask. I am in desperate needs for a xtra thick mattress topper, for a single bed. My son has been told that he needs to get rid of his spring mattress as that is causing him pain in back and shoulder.
He does like the firmness of his mattress, however he can’t deal with the muscle pain.
I can’t afford new mattress or topper at the moment, just wondered if anyone could help at all please. Must be close as I do not drive. Thanks in advance 😊