2009 Impala SS
I thought belt molding is the outside window sweep and trim all rolled into one.It is called a belt-line molding. It is separate from the rubber molding that attached to the top, inside of the door panel.
That's the actual door gasket. GM would probably sell it to you for both kidneys and your firstborn.
You may be able to liberate one from a wrecked Impala in the Pick N Pull. I've picked up belt moldings and other trim like that.