Couch back coushion upholstery?

I have couch which has removable 2 seats coushions and 2 back coushions.
The seats coushions are different from the back coushions. Each seat coushion is made of a single sheet of good quality foam which is coverd by a fabric with zipper, so its washable.
But the 2 back coushions are made of something like cotton stuffings (not a foam sheet) and the fabric does not have a zipper hence not washable.

Now my problem is even though my couch is in a good condition the back coushions have lost its shape and stiffness leading to back pain.

I’m thinking of replacing the back coushions similar to the seat coushions (foam covered with washable fabric with zip).

I have found the exact fabric in Joann’s. But I dont know how to purchase the foam and stitch the fabric cover with zip. Suggestions required. Should i good to a upholstery? how much would it cost? will they have the foam or is there another shop where we can choose our foam?

Call any seamstress in the phone book. This is a job they do all the time. Ask them what they charge for labor. They will tell you, based on your cushions, what, and how much you need to buy. and, just because the cushions have a zipper, doesn’t mean the covers are washable.