To replace sagging couch cushions, re-stuff the original fabric with fresh foam batting or sew a new set of covers based on the dimensions of the original cushions. In some cases, it is the couch itself rather than the cushions that cause the sofa to sag. Slipping a sheet of plywood underneath the cushions might solve the problem without having to replace the cushions.
To ensure aesthetic appeal, choose a matching fabric if the cushion covers must be replaced. This is easier with solid-color sofas than with patterned fabric.
If the original cushion covers are in good shape, reuse them by removing the foam batting or sheet from inside them and replacing it with fresh foam. Zip up the cushions and slide them back onto the sofa.
Sew new cushion covers
Measure the panels (top and bottom) and the sidewalls of the original cushions, then cut fabric to those dimensions, leaving an additional one-inch seam allowance for one sidewall to accommodate the zipper. Sew the pieces together in a box shape, then sew the zipper into one sidewall. Fill the cushion with foam batting or sheets, then repeat for any additional sofa cushions.
It is also possible to purchase replacement sofa cushions. Many companies offer cushions in custom sizes and shapes.