The Bible does not explicitly mention where Cain found his wife. This question has been the subject of much debate and speculation among biblical scholars and theologians. However, there are a few different interpretations and theories:
Siblings or Other Family Members: One common interpretation is that Cain married one of his sisters or another close family member. In the early chapters of Genesis, it is mentioned that Adam and Eve had other sons and daughters (Genesis 5:4), so the assumption is that Cain married one of his sisters or a niece.
Other Humans Created by God: Some suggest that God may have created other humans in addition to Adam and Eve. This theory proposes that Cain found a wife from this separate group of people.
Nephilim or Pre-Adamite Humans: A minority of interpreters suggest that Cain might have married a member of a group called "Nephilim" mentioned in Genesis 6:1-4. Others propose the existence of pre-Adamite humans, suggesting that there were human-like beings living before Adam and Eve.
It's important to recognize that the Bible does not provide a detailed account of early human history, and certain gaps and ambiguities exist in the narrative. As a result, different interpretations and theories have emerged over time. The focus of the biblical narrative is primarily on the relationship between humanity and God and the theological messages it conveys, rather than providing a comprehensive historical record.