Aluminum window trim: what is it called and where to look for it?
That looks like powdercoated steel.  In that size, you probably will have to get something custom made.  You can try to source the raw powdercoated steel yourself online (or locally?) and have a professional shape it for you.  If you are lucky you might find some pre-bent 90 degree stock online as well.  

Here's one possible one-stop shop, but it might be pricey.  Be careful to match the sheen to your existing metal! http://www.rapidsheetmetal.com/powder.aspx

I think you'd be best off finding a fab shop locally so you can bring your piece in and just have them duplicate it to your specs.