I gave up on it after a long search. Like good diet grape soda, it is my Unicorn.
You don't actually need much for a Dark n' Stormy, I usually give up my carbs at dinner if I want to treat myself with a Dark n' Stormy for dessert. (From a diabetes standpoint, you might be looking for diet for a completely different reason.)
Or, as Toby from Violet Hour suggests
over here, you can make your own Ginger Syrup (with an artificial sweetener, of course)