Not every organic product has to come from an isolated farm, far away from the city.
The Brooklyn Grange is the main green roof project in the USA. They simply operate the largest farms on a rooftop in the world, located on top of two buildings in New York. There, over 50,000 pounds of organic products are grown annually, which means that many vegetables and herbs in the city come directly from there.
The Brooklyn Grange also provides consulting services on urban agriculture and the installation of green roofs. All in all, they help keep local communities well informed and healthy
See here for a list of markets where you can buy vegetables grown in Manhattan's backyard.