eBay. That card is long since discontinued and no longer supported by AMD nor Microsoft.
I have had good luck by removing the full height bracket from the card take a chisel and hammer and crease the bracket where I want to bend it to make it a half height bracket, then drill a whole where the screw mount will be. I then put it in a vise bend it over and then use tin snips to cut off the excess. Not pretty but works well.