It's unlikely that these are sold in bakeries here in Philly's Chinatown. You can find mantou, which are made from the same dough, but are a different shape, sold in most Philly Chinatown bakeries (look in the steam racks or ask). I'd buy those and then cut them down to size. Chemicalkinetics offers a good suggestion of where else to look.
Alternatively, they can be made at home as tfalbo suggests. They are not difficult, just time-consuming.
Finally, you can buy frozen mantou which are easily reheated with a steaming. Same problem as they are not the same shape.
Please update if/when you accomplish this!
I have been to Philly Chinatown many many many times (every weekend), but I have not seen Momofuku pork buns in these Chinese bakeries.
I honestly think your best bet is not the typical bakeries, but the noodle factories 麵廠. In other words, stores which sell special noodle, rice noodles, buns...etc.
i know this answer is completely ignoring your request, but just in case no one else on the board can help you source the buns pre-made (unfortunately I am no help on that front), I actually tried making them for the first time last night, and it was significantly simpler than I was expecting:
Like I said, it was my first time trying, and, while not perfect, i got significantly closer than I was expecting. Hopefully someone can point you to the buns premade, but if not I wouldn't turn away from the task of making them yourself.