IMO, I would make my own. Easier that way
lol 
and sometimes cheaper.
What i would do is find some heavy duty light weight tubing and have someone bend the tubing 90 degrees like in the photo. The reason i would use tubing is because it disperses the weight better and you shouldn't have to worry about it. That depending on how heavy the light is though but you can always add another bracket or two in the middle
I think you might be able to find stainless steal tubing about 1/2 inch to 3/4 inch at most steal shops and they might be able to bend it for you when you buy it. Also depending on the tube i would have them drill the mounting holes as well. Some steal or material a little weaker than steal can be a pain to drill. If they drill it then they should have a drill press with a clamp to hold the tubing and make it a lot easier.