I have never seen plain onigiri here. There is yakionigiri in the frozen section with no fillings.
Today I went around at 10am and I bought several of the individual onigiris. I didn't see one that was plain if you wanted the pre-made onigiri. You might have to buy the rice and cook it yourself or ask a staff if they have any in stock.
They have sushi right so I think they should have it
They sell uncooked rice, they also sell to-go onigiri rice balls with no fillings
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