Fiqh-us-Sunnah Volume 5, Hajj and ‘Umrah, Fiqh 5.110B.

Sacrificing a camel is not necessary except for a pilgrim who performs tawaf in a state of ritual impurity (Junub) or a female pilgrim performs tawaf while still menstruating or in confinement after childbirth, or a pilgrim who has sexual intercourse with his wife after spending the Day at Arafah but before shaving (or clipping) the hair, or a pilgrim who vows to sacrifice a camel; in all such cases one must sacrifice a camel. And if a camel is not available then offer seven sheep instead.

Ibn ‘Abbas reported that a man came to the Prophet (peace be upon him) and told him that he had to sacrifice a camel which he could easily afford but could not find. Thereupon the Prophet (peace be upon him) said to him, “Buy seven sheep and sacrifice them instead.” This is reported by Ahmad and Ibn Majah with a sound chain of narrators.