Seaweed
Dried seaweed, in Korean cooking mainly miyeok (wakame), gim (nori) and dasima (kombu), with a salty, briny flavour.
Caution: Kombu and other brown seaweeds are very high in iodine and should be eaten only in small amounts rather than in large daily portions.
Dried seaweed is reliably found in Asian grocers or by mail order, nori sheets often in supermarkets too, and wakame and kombu sometimes in organic or health food shops. Mind the variety, since miyeok for soup, gim for wrapping and dasima for stock behave very differently in the kitchen.
Substitute:
There is no real substitute outside seaweed, as nothing else provides the briny flavour and texture; Japanese wakame, nori or kombu are the same seaweeds as the Korean ones and can be used directly.