If a memory foam mattress is too expensive, try a 2 or 3 - inch memory foam topper instead. (thespruce.com)
Because of the extra memory foam topper and mattress cover though, it made is very hard to find sheets that would actually stay on the mattress. (thefashionablehousewife.com)
I eventually got a 2 inch memory foam topper and a mattress cover to put over the whole thing. (thefashionablehousewife.com)