I was wondering where can I find the Kartboy Mounting Bracket for the Hella 500 lights...

You can try straight from kartboy.com. Although you could go to rallylights.com and order a set from them. I have a set and I must say they are great.

http://www.drunkhourphoto.com/uploads/IMG_0608.jpg

http://www.drunkhourphoto.com/uploads/IMG_0616.JPG

http://www.allwheelengineering.com/basket/catalog.cgi?query_net_id=kartboy&query_class=KB-010

oakos.com used to sell them as well. but tom at kartboy.com can help you directly.