Where did Cain's wife come from?

Genesis doesn't say directly, but it gives the pieces needed to answer reasonably: Cain most likely married a close relative, probably a sister or niece.

Genesis 5:4 mentions, almost in passing, that Adam "fathered other sons and daughters" beyond the sons named in the story. Since Genesis presents Adam and Eve as the first two humans, their children and grandchildren marrying each other would have been the only option in that first generation.

This can sound strange by modern standards, where marrying a close relative is illegal and medically risky. The Bible itself later prohibits close-relative marriage once humanity had grown (Leviticus 18). Many Christian thinkers suggest that in this earliest, genetically healthier population, the genetic risks we associate with this today simply didn't apply the same way yet.

It's a fair question, and the text doesn't hide from it—it just assumes a starting point ancient readers wouldn't have found strange, even if it strikes modern readers differently.