Sour orange juice
Juice of the bitter orange (Seville orange), sour like lime, with a tart, bitter undertone and little sweetness.
Bitter oranges are best known as the marmalade orange; fresh ones usually appear only in winter, mainly in January and February, at well-stocked greengrocers or markets. Bottled juice is available year-round in Latin American shops or by mail order, often under the Spanish name naranja agria.
Substitute:
Two parts freshly squeezed orange juice to one part lime juice match the acidity and fruitiness well; a little grapefruit juice or some grated orange zest brings back part of the bitterness, though never all of it.