Where to buy new cast iron radiator?
You can call U. S. Boiler company Lancaster, Pa. 888-432-8887 or 717-397-4701 or  "GOOGLE" U S Boiler and tell them what you want. They can tell you where to buy the radiator. You can also "GOOGLE" used cast iron radiators. New radiators are quite expensive so I would have a steam heating service company check the existing rad to see just what is leaking on that rad. Depending on where or what is leaking, the rad may be able to be fixed.