A popular ingredient throughout Japan. Here in Okinawa, there is a local specialty miso called andansu アンダンスー, but it can also labeled 油みそ or even 肉みそ for the pork based or かつお for the katsuo based. It is miso with bits of pork in it, or sometimes katsuo かつお (鰹, tuna type of fish). You can find it in local grocery stores and farmers markets, although it is not always near the other miso pastes… sometimes I find it by meats or prepared foods, or even in the “local specialities” section. At farmers markets it is often sold in simple containers or even just plastic bags (usually from local obaasan that make it at home to sell), but most grocery stores sell the commercial versions. You can even make your own andansu of you live outside of Okinawa (recipe here).
Obviously this is not one I go for since I do not eat pork, but my husband likes it.
Andansu is used in several dishes or even just as a condiment on rice. A popular treat called ぽーぽー popo, sort of a rolled crepe made from eggs, flour and brown sugar, puts a bit of andansu in the middle.
So as a reminder, not all miso is vegetarian, however if you don’t mind a bit of pork, you should try some andansu while in Okinawa.