Rice is everywhere and practically everywhere rice is consumed and onigiri is Japan's exclusive gift to the world of rice.
One could object: what about sushi? Of course, it is also Japanese, but sushi as we know it today rather represents the sublimation of something that, unlike the onigiri, was not exactly born in Japan.
For such reason, the onigiri is an institution and as soon as you have landed in the archipelago, it is worth going in search of the best places to savor it - perhaps after tasting one in a conveninent store whatsoever: Seven Eleven, Lawson, Family Mart, almost everywhere nobody denies you an onigiri on the fly and you can immediately get an idea of what is the (already very tasty) base from which you start.
After that, a world opens up. We recommend a trio of onigiri-ya in Tokyo to start from, then the sky is the limit, as they say.