Best mattress after back operation.

I had a back operation a few years ago. I'm beginning to have twinges again when I wake up in the morning. My current mattress is cheap and saggy.
I have a slatted base on an old iron bed with brass knobs. What do you suggest. It's 4'6" wide.
I weigh 175 lbs and am 5'7" tall, aged 64 female.