I spent 3 years in Japan, and there's no downtown place besides Sanko that carries certain Japanese specialties I happen to love. Besides all the usual supsects (sushi-making supplies, ceramics, comics, noodles, edamame, miso etc.), they have wonderful harder-to-find goods like tsukemono (all the yummy pickled things), umeboshi (pickled plums), Japanese-style onigiri (formed rice balls with various middles), and the best soft drinks. I also love the automated greeting recordings.
They have an automated greeting machine at the door that makes you feel just like you're in Japan. They have quite a few delicious fresh and frozen sweets and ingredients as well as a few different flavours of onigiri for a quick snack.
it's small, but a great place to get your sushi supplies and prepared japanese dishes.