My neighborhood supermarket, which has only a modest selection of Jamaican products, has it. It comes in cellophane packages w/ cardboard hangers on a rack along with spices etc. I'd try any market in a WI neighborhood.
There's a Bronx company that specializes in bottling Caribbean drinks. I can't remember the name, but I definitely saw bottles of Irish Moss at Christie's Jamaican Patties on Flatbush Avenue in Brooklyn.