Just for info, I just bought a special size [71" x 79"] king
mattress 13" thick Englander with latex top and base springs
for RM4k from Harvey Norman at The Curve. This is a firm
back-care mattress.

At a so-called factory outlet in S19 PJ I was quoted RM5.3k for
the same model so it pays to shop around and not fall for super
sales/special clearance pitch.