First step is always Chad at the factory. If no luck there, try Spruce, Wag Aero, Univair, Ebay, etc.
Are you sure you are supposed to have a bracket there? That is common on Citabrias and Champs. However, every Decathlon I have seen has a hinged brace at the lower front edge of the window to limit how far it opens. Max speed with the window open is well below typical Decathlon speeds.