The best source I've found for the black metal grill material is one type of .... 3-tier stationery tray. No kidding! I've picked two of these things from the scrap metal recycling place, each with three strong, stiff trays that can be cut to size, even leaving a bit of one folded edge at the back, if desired. They are long enough and wide enough, too. I've only found one type that is good for this use, but that type is really very good. Strong, well-built stuff that won't bend easily, has plenty of holes (resembles the original) and is pre-painted black. The paint used will take the heat, too.
Unfortunately, I don't know the brand or where to get them, because of where I got mine. I just keep my eyes open when I go by there.
You might be able to find something close from a good hardware store, Home Depot, etc...