Time Out says
Don't expect dainty bites here – the onigiri at Marutoyo come in one size, and one size only: large. The store carries a wide assortment of rice balls, all filled with Tsukiji-appropriate fillings, including roasted fatty mackerel, shiokara (salted fish innards), nintama miso, oyster, octopus, and clams. The one that's most likely to satisfy, though, is the shachi-ten, a rice ball filled with a whole fried shrimp.
4-9-9 Tsukiji, Chuo-ku
|Transport:||Tsukijishijo Station (Oedo line)|
|Opening hours:||5am-3pm / closed Sun & market holidays|