Onigiri with elaborate ingredients at a specialized store.
Traditionally, an onigiri is filled with common ingredients, for example pickled ume (umeboshi 梅干し), salted salmon (sake 鮭) cod's roe (tarako 鱈子) and small pieces of sliced dried bonito (okaka おかか) and such common onigiri can be easy to get at convenience stores. But stores specialized in Onigiri stock ones filled with various elaborately cooked ingredients.
Today I bought a Onigiri filled with soft-boiled egg with a taste, with Shrimp Tempura and wrapped with sliced pork at the food market in a department store basement in Shinjuku. They are so troublesome to cook with many processes that I hardly do them at home and simply buy them.