Sushi is the traditional and most known Japanese food abroad. In Europe, sushi can eaten at running sushi restaurants or Japanese restaurants. On the other hand, sushi bars and take away sushi are limited. May be you can buy sushi at supermarket but it is made some away else the freshness is unclear.
In Japan, it is much easy to find sushi at supermarkets. There are also stores specialized in take away too. You can find these stores in stations or in department stores and also in shopping streets. If you are alone or just want to eat the meal at your accommodation, it is very ideal to buy the sushi for take out. Of course the prices are much reasonable than eating at the sushi restaurants.
The shop assistant will ask you how many chop sticks you need and of course soy sauce and wasabi will be accompanied. Just watch out about the best before date and time. Sushi must be fresh and no preserves are used, it should be kept in the fridge no longer than the same day.