Can you think of any particularly worthwhile supermarkets in the areas readers here are most likely to visit, such as Tokyo, Kyoto, Osaka and so on? It may help readers - even short-stay tourists - pick up a bargain, or even just a flavour of Japanese daily life.
(I hasten to add that if someone were coming to the UK, I wouldn't rush to reccommend they hit the local Tesco, but you never know what might interest someone else
)