How

a job like that should be done by a professional . I don't know of any company that sells a replacement for that metal . most shops make them as needed from sheet metal. the rust has to be cut out and new metal welded in. if you have a welder you can go to any body shop and ask them for a few pieces of metal i'm sure they'll give it to you. a sheet metal brake would be useful for bends and a shrinker / stretcher tool for curves.

the cheats way is to get some wire mesh and fiber glass the area,and build it up gradually till you have the apropriate shape and lip for the window seal to sit in it is a laborous task which will take time but if done right and the rust taken out and genolited will last several years