Long shot, pull the pins and carry the seat to an auto trim place or furniture re-upholstery place, just for a cover should be reasonalble . Cartouche usually will not sell incomplete sets, But my car got a stain. They sold me all the stuff for just the front seats for $300.
Next idea, present it to your local Senior Center and see if they have a sewing place.
Last choice, I am making slip covers for my car, I am using a canvas type material, just for a dust cover and to drive when I am in my work clothing. I can see ho much a second set it. I.M. me if you are interested. If you pick out the fabric, she can tell you how mich to buy. These are slip covers, but could make them look reasonable. She is very reasonable, and while she is cutting one, it's just as easy to make 2.