Put the dry beans in a bowl of very hot water and set aside. Turn the oven to 400F. Salt the turkey drumsticks well on all sides and put them in a roasting pan; you can coat them with a little oil if you'd like. Roast, turning once or twice, until browned, about an hour.
Meanwhile, in a large soup pot, add the turkey broth and 4 more quarts of water. Bring to a simmer and add salt to taste. Drop in the dried corn and simmer this while the turkey legs roast.
When the turkey legs are ready, nestle them into the pot and add a bit more water if they're sticking out of the water. Bring to a simmer and stir in the oregano, chipotles and ancho powder. Simmer gently, partially covered, for 2 hours.
Drain the beans and add them to the pot; they'll usually need an hour to cook.
While everything is simmering, heat the lard in a frying pan and saute the onion over medium-high heat until the edges brown. Chop the roasted chiles into pieces you'd want to eat in a stew and add them to the frying pan. Saute for 2 minutes, stirring often. Add the contents of the frying pan to the pot.
When everything is getting tender, fish out the turkey legs and lay them on a baking sheet to cool. When they are cool enough to handle, strip all the meat off the legs and add it back to the pot. Discard the bones.
Finish with the chopped cilantro and serve with sour cream on the side.