There are not many places left in the world where classic propliners are used commercially. Alaska with its rough weather and runway conditions is the place where these planes are still in the air every day.
The biggest Douglas DC6 operator in the world is Everts Air. Everts is using them to transport any kind of cargo and fuel to the remote cities all over Alaska. Fuel is also hauled by the nearly indestructible Curtiss C46. Spare parts are not a problem at the moment, but there is the end of the high octane fuel on the horizon. That will probably be the reason for the end of the classic propliner operations.