Rice vinegar
A mild, pale vinegar made from rice, less sharp and slightly sweeter than wine vinegar.
Rice vinegar is sold in Asian grocers and on the Asian shelf of many supermarkets. Look for plain rice vinegar, since seasoned sushi vinegar already contains sugar and salt.
Substitute:
Cider vinegar or white wine vinegar, diluted with a little water and rounded off with a pinch of sugar, comes close but tastes fruitier.