Are you talking about Nikuman? Nikuman are japanese buns that are actually modeled off of a chinese bun called Char Siu Bao, which you can find all over Chinatown, in various bakeries or Dim Sum restaurants.
If you're talking about the mantou pork buns that have become popular at modern ramen houses like Ippudo and Momofuku, I don't believe those can be found in Chinatown. In either case if you are looking specifically for Japanese food you should probably look in the East Village or Midtown rather than chinatown.
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