I found a reliable bahn mi source in Pho tu do, at 119 Bowery. There is a small bar window where they are sold in the entryway. They only have two varieties but for 2.75 the Saigon Bahn Mi is filled with lots of sweet crumbly pork. The only shortcoming was the bread was a bit over toasted though and made it difficult to eat. For a dollar more you can upgrade your cold cut filling to bbq pork slices.
The same exact sandwiches appear to be sold saran-wrapped next door at the Century Cafe, a Chinese bakery. They are labeled "French Sandwiches" and are piled up next to the register.