sour cherries
Small, dark red cherries with a pronounced tartness, used in Persian cooking mainly for rice dishes and syrups.
Fresh sour cherries appear only briefly in summer, but pitted frozen ones or jarred morello cherries are found in many larger supermarkets, and dried ones in Turkish, Persian or Middle Eastern shops. Drain jarred cherries, as they are sweetened, and keep the juice for syrup or sauce.
Substitute:
Sweet cherries are not an equivalent substitute, because they lack the acidity the dish relies on. A little lemon juice only gets you roughly there, and soaked dried sour cherries are the better choice.