The pork bun is available at the "regular" Momofuku at 163 First Avenue between 10th and 11th Streets at lunch. I remember reading a while back that it was only available at Ssam at dinner (which may have changed, although the online menu indicates that it is still the case). Ssam is at 2nd Avenue and 13th street, which is a very short trip from the original Momofuku. If you need to go to Ssam, start at the original with a "warm up" order of the pork buns. Do not be tempted to share an order with anyone else as this can lead to very ugly territorial behavior. I like to follow up my pork buns with a trip down the street to Rai Rai Ken at 214 10th street between first and second for a bowl of ramen. I find the ramen there to be superior to the ramen served at Momofuku (and very inexpensive).
Whatever you do, don't miss the pork buns (or the soup dumplings at Joe's Shanghai). Best of luck.