Though the Bible does not answer this directly, it does tell us enough to draw an informed conclusion.
Adam and Eve were the first and only people on the earth right after the six days’ work of Creation. They were commanded to be fruitful and multiply and replenish the earth (Genesis 1:26-28). They had not only Cain, Abel and Seth but numerous sons and daughters (Genesis 4:1, 5:4, 6:1) over a lengthy lifespan (Genesis 5:5).
We know Cain was over 100 years old when he took his wife and went into the land of Nod. At that time enough people already lived there to build a city. There he started his family (Genesis 4:16-26). The Amplified Bible translates Genesis 4:17: “And Cain’s wife [one of Adam’s offspring] became pregnant….”
The fact that Genesis 3:20 says, “Then the man—Adam—named his wife Eve, because she would be the mother of all who live,” is proof enough she was ultimately the one responsible for Cain’s wife as well as for Cain himself. The first marriages had to be between brothers and sisters in order to get the race started. After that, marrying close relatives was forbidden (Leviticus 18).