The Garment District is a neighborhood of the New York City borough of Manhattan, located between Fifth and Ninth Avenues from 34th to 42nd Street.
The Garment District is the fashion center of New York City. Although hardly one square mile, this small district, anchored by the Javits Convention Center at the extreme west, the New York General Post Office, Penn Station, and Madison Square Garden in the center, and the Empire State Building in the east, contains an extraordinary concentration of industry. The neighborhood is home to the warehouses and workshops of the fashion industry.
** just click the source and you could get directions from there
Silk screening may be your next best choice, depending on your designs. But that can get expensive if you are only making one off shirts.
How artistic are you? Perhaps hand painting/printing your shirts is the way to go. With acrylic fabric paints, markers, and inks, you can do just about whatever you like. You could add tie-dying or dip dying to make each shirt unique. I know Michael's Craft stores carry a variety of colors of Jerzees brand shirts, which are often on sale.
The links below are for ideas for creating your own tee shirt designs, and the Joann link shows many of the paints available.