I have never seen the panels sold separately. I am not saying they aren't available, I have just never seen them.
If your shower is white, I might suggest FRP (fiberglass reinforced panel). It gets used in a lot of commercial kitchens and should hold up very well for your use.
I don't know if i'd go with the wallpaper. I have a small shower room in my home, and I likes me a hot shower. The paper is the first thing that you notice when you walk in. Edges curl, seams separate, it's ugly.