Options for current armored cable 12/2 but need 12/3
First off, you can not use the green, ground, wire as a neutral. You are right about not being able to use NM cable in NYC. It is allowed in certain residential applications in other parts of NY but not in the city. 12/3 with a ground armored cable is what you need. Most home stores will sell a 25 ft reel. 14 gauge wire is also not allowed in the City so don't try to bring some in.