IIRC, the whole wall panel on the side above the floor is one vacu-bonded panel with the cutouts for the slides and windows. I'm not sure you can buy a piece of it; you will probably have to repair the spot that's damaged, and would agree with E-Mike that you probably need to find a repair facility that knows how to repair vacu-bonded panels.
We have been fighting a leak over that driver's side window for a long time. Finally figured out ours was coming from two places: 1)the area above the window where the horizontal lap joint is created where the wall panel joins the roof cap (re-caulked with clear silicone caulk), and 2)the area where the front cap joins the roof over the steering wheel. Fixed that at Quartzite with some new caulk across the whole front end cap joint. We have a flat spot in the roof where water pools behind that front cap over the driver seat/steering wheel, and water was running in there. Both places were leaking, and the roof cap/front cap joint was leaking into the roof cap/wall panel joint above the driver's side window.
Don't know if that helps, but that's what was causing our leaks.
Former Owner, 2006 36MDDS
2010 Arctic Fox 22GK Travel Trailer
2007 Toyota Tundra