I used to be able to find Heineken Dark on occasion in my neighborhood store but i called and went to dozens of places near me and no one carries it anymore. I live in NJ shore area. Did they stop making it???
I can see it on Beermenus, so it's around. Maybe your regional distributor stopped carrying it. Have you tried any other substitutes, like Negro Modelo, Czechvar Dark, Baltika #4, St Pauli Girl Dark, DAB Dark, etc? If so, what did you think of them? It might help us find you something else to enjoy.
My brother-in-law was a Beck's Dark Drinker until it was discontinued. His replacement was Warsteiner Dunkel, FWIW. Says it's the closest he could find.