Where and how did Cain find his wife?

Many persons teach that Adam and Eve and their children were not the only human beings alive, and that "other persons" lived elsewhere on earth who were not mentioned in the bible.

This teaching is not found nor supported in the bible at all. 

They use as proof these verses Genesis 4:16 "And Cain went out from the presence of the Lord, and dwelt in the land of Nod, on the east of Eden. 17 And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch."

They imply that it was after he went to Nod that he got a wife, but the bible never said that. It is most likely that he had his wife prior to going to Nod however it was while in the land of Nod that she became pregnant (Cain “knew” his wife). His wife would have been a sister or some other close family member.

The reader must always bear in mind that the bible authors only mentioned the main male characters in the bible. In this light, Adam and Eve had many other children as they were commanded to be fruitful and multiply and replenish the earth (Gen. 1:28). Cain and Able were adults and the eldest males and the objects of their story. They likely had many other brothers and sisters who were also adults themselves. Seth was born after the death of Abel and He was the one in whom the family line would be passed down through and he was the “spitting image of Adam (Gen. 4:25, 5:3). The children and grandchildren of Adam and Eve were to find husbands and wives from among themselves as they were the only set of human beings. Yes, I know that this might sound disgusting to you but what are the options in a world of only one family?

These close intermarriages were supposed to end once the world population had grown substantially, allowing the relationships to then be between very distant cousins. Leviticus 18 reminded the Israelites of this fact and condemns incest because there was no longer any reason for such acts. 

Please note however that we all are still one family and that each male and female that are married are still married to their distant cousins.