Source for Chinese steamed buns?
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.
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.
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!