The window will be common to the whole Challenger range for that year and possibly other years and models. You do need to measure it first and then breakers can use these to check if what they have fits. You could try SFM Caravans of Halifax as they stock large quantities of parts from scrapped caravans. The Challenger was always a popular range so hopefully you can find a second hand one, but if not try EECO who can make you a new one, but at a cost.