That looks like an angle bracket.
The large sizes like the one in the picture are commonly supplied by builders merchants and probably the likes of Screwfix and Toolstation etc.
However I would of thought the bracket in the photo is not a standard of the shelf product as that looks like it has been made for the purpose or at least modified to suit.
You might even have to get one made by an engineering company, aluminium might just be suitable, or it would need to be galvanised or better stainless steel otherwise it will rust and perish in a short time.