If you're very lucky you might be able to find a matching alloy wheel from a scrap yard, but since DS 3 has such a breadth of customisation options and your model was a special edition it's not particularly likely. So you'll probably have to go via Citroen - though if you can identify the pothole that caused the damage you may be able to get compensation from the council responsible for the road, so keep all your receipts to forward to them.