We’re told Adam begot Cain and Abel, then afterwards he begot Seth and had other sons and daughters also. In the book of the genealogy of Adam, Genesis 5, we are also told "he had sons and daughters," Genesis 5:4.
Moses as author didn’t bother to record when daughters were born, no wonder people question where Cain got his wife, but neither does the Bible tell us where Seth also got his wife either. Later in life it was taboo to marry your own sister or brother, and this is the reason why bible translations edited that out, but it was not an issue then.
"This is the book of the generations of Adam. In the day that God created man, in the likeness of God made he him; Male and female created he them; and blessed them, and called their name Adam, in the day when they were created," Genesis 5:1-2
In Genesis 1:27-28, "So God created man in his own image, in the image of God created he him; male and female created he them. And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth..."
Where did Cain get his wife?
"And Cain went out from the presence of the Lord, and dwelt in the land of Nod, on the east of Eden," Genesis 4:16.
The east of Eden, meaning not too far away, is what Moses’ generation knew as the land of Nod. Otherwise Cain and probably his wife, were the first people there.
Who then was his wife?
Remember, Adam, "he had sons and daughters." Genesis 5:4
God bless you,
www.muzembi.org - Simple Gospel Summation