Where can I find a waterproof couch cover for dogs?

My dad’s small dog sometimes pees in her sleep, causing accidents on the furniture. The vet thinks it’s due to her old age and a weak bladder, despite being otherwise healthy and well-trained to go outside often. The vet suggested diapers and pee pads, but they haven’t worked well.

My dad lives in a small, open apartment, so keeping her off the furniture is challenging. He has to crate her when he can’t watch her closely, which he prefers not to do for long periods.

I want to find a cover that will protect his sofa from her accidents. Most sofa covers don’t have a truly waterproof bottom layer. I’m considering getting a large sheet of plastic or vinyl instead. Any suggestions would be helpful.