Beef Suet in NYC?
I've never tried to purchase it, but I suspect many good butchers in the city would have it. If you don't have any certain responses, call a Citarella, Ottomanelli's or Florence Meat Market (my personal favorite).
Also some leads in this older thread: http://chowhound.chow.com/topics/672444
424 6th Ave, New York, NY 10011
Ottomanelli and Sons
285 Bleecker St, New York, NY 10014
Florence Meat Market
5 Jones St, New York, NY 10014