Rust repair around rear window

[jae]

My brother just bought a '73 and didn't wait for me to go with him and apparently he didn't think the rust issues were as bad as I do. Anyway, on the driver's side, lower corner of the rear cab window, there's a fairly bad rust spot but I can't seem to find any place that sells a patch panel for this area.

Can anyone point me in the right direction, or would I just be better off going to a salvage yard with a Sawzall and cutting out a piece of a cab there?