You might try O’Learys who run a caravan and motorhome scrapyard near Hull. They are on 01482 868632, or Leisure Spares in Boroughbridge , N.Yorkshire, on 01423 320009, as they do Dometic/Seitz roof light spares. There is also The Caravan Centre in Blaenavon, 01495 792700, or EECO Ltd, who make windows to order. Caravanwindows.net in Kidderminster on 01299 250273 or Nationalcaravanbreakers.co.uk on 01492 512880 might also be worth a go. I haven’t used most of these people myself, and I don’t know if they are working at the moment, but members have mentioned them.
We had a window problem with our previous van, in that a barn jumped out and cracked a side window, and it took me 18 months to get a replacement. Hobby UK in Preston finally had one made for me, but it was the devils own job to get them to do it, and we drove around for all that time with a window covered in gaffer tape.
Hope this helps, and good hunting