You should see a doctor, preferably a cardiologist, to discuss possible alternatives/substitutes for Nikoran OD. He/she will be able to give you the proper dosing, make sure there are no contraindications with your other co-morbidities, and make sure there are no unwarranted interactions with other drugs you are currently taking.
These are similar but not exactly the same mechanism of action.
Diethylene glycol dinitrate
Isosorbidemononitrate and dinitrate
Nicorandil (which additionally acts as a potassium channel opener)