Vinegar
A mild, faintly sweet vinegar made from rice, much gentler than wine vinegar.
Many supermarkets carry rice vinegar on the Asian shelf, and Asian grocers or online shops reliably stock it. Check the label, because seasoned rice vinegar (sushi vinegar) already contains sugar and salt, which changes how you season the dish.
Substitute:
Light cider vinegar or white wine vinegar, thinned with a little water and rounded off with a pinch of sugar, comes close. It tastes slightly fruitier and sharper, and the difference shows most in sushi rice and delicate dressings.