Mara's Homemade has them. I think* they do a steamer type deal. I know they definitely do them in other Creole dishes. They also have relativelt cheap pints of Abita beer and an awesome bourbon pie- I highly recommend it!
342 East 6th Street, New York, NY 10003
Fresh or frozen? A boil or is it OK if it's something else? I thought the season peaked in spring (May or so), because that's when the few NYC restaurants that DO serve them have their crawfish festivals. I'm afraid that September might be too late in the season...
In 2009, places to get crawfish in May: