Kimchi
Korean lacto-fermented vegetables, usually napa cabbage with chili, garlic and ginger: sour, spicy and pungent.
Kimchi is sold in Asian grocers and increasingly in the chilled section of larger supermarkets, in jars or pouches. It grows more sour the longer it is kept; for cooked dishes, older, distinctly sour kimchi usually works better.
Substitute:
There is no true substitute; in cooked dishes, raw sauerkraut with chili flakes, garlic and a little fish sauce can approximate the sourness and heat, but the characteristic depth and crisp cabbage are missing.