A home brew shop would stock it (it's used to clarify wort) but not sure there are any in Manhattan though there are several in outer boroughs...might try the "spice" stores such as Sullivan Street Spices or Kalutsyan...
123 Lexington Ave, New York, NY 10016
Sullivan Street Tea & Spice Co.
208 Sullivan St, New York, NY 10012
Hey, I'm just a newbie here on CH, am merely a visitor from Boston, and might be pointing out the quasi-insultingly obvious, but I would trek to the "most" Jamaican neighborhood you have access to & hit up a retailer/bodega/W. Indian specialty store. The places I know of that'd be apt to sell it in BOS are all "in the hood", and frankly only know of Irish moss through JA friends. Hope that gives you a starting point. Big up!