Cut a piece of plywood or MDF to fit under the cushion. The problem may be the lower structure and not the cushion.
I refilled my sagging chair it with more stuffing that is used for pillow . You can buy it on amazon.
Lisa is there a zipper where you can get to the cushion inside? If so that is easier and you can add memory foam bedding cut up to fit your cushion. Just place it inside and straighten it out. I use memory foam bed toppers all the time for my re-upholstery projects as well. It can add a little oomph to the old cushioning. good luck.
Our friends at church had this problem and they laid a piece of plywood from the metal frame to the metal frame under the cushions and that fixed it. Perhaps this might help your couch?