Probably in a lot of places, there good handful of Japanese grocery/convenience stores in Manhattan and the Korean places carry a lot of Japanese things. The most likely are the two Sunrise Marts (East Village and Soho) and Katagiri on E 59th St.
I find that it is very difficult to find. I use to find it consistently in Costa Mesa at Mitsuwa. I think there is one in NJ. I would only want to make the trip if I knew it was there and not available anywhere else in NYC.