Where did Cain get his wife?
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).