Umeboshi (梅干, うめぼし) are pickled ume plums common in Japan. They are a very tiny fruit, about 1 inch in diameter, and are pickled with the pit inside, so be careful with your first bite! Umeboshi are a kind of tsukemono (漬物, つけもの, pickled thing) and are extremely sour and salty. Umeboshi are prepared by drying the plum, but can also be prepared without drying. Those plums are called umezuke. (梅漬け, うめずけ)
Like ginger, the plum acts as a preservative, which is why it is often packed with rice, usually placed in the middle to represent the Japanese flag. It is most commonly enjoyed in onigiri, but can be enjoyed plain or with green tea. It also has antiseptic properties, is claimed to help prevent nausea, and help with hangovers!
Unfortunately there really is no substitute, so if you can’t find it near you, you’re going to have to buy it online. It usually isn’t required for any dish, just an option, so you won’t ever have to buy it, but you might be curious about the taste. But since it is a pickled food, it has a long shelf life, so you can buy it in bulk and it will keep for quite some time! In fact, some people like to eat aged umeboshi for the taste.Buy on Amazon