I had to get a replacement window recently. Twin wall plastic for my Knaus. I ended up buying new from camperbros.com. Some advised against it, but the quality is OK and so far so good. OG replcaements were starting to push beyond €800 and got one quote in excess of 1000! It's for the over cab bunk, a smaller window, and doesn't get much use. Full window kit, hinges, struts, etc delivered for around €230.
You might get lucky with a breaker, but I didn't