Where did Cain get his wife?
Cain's wife was his sister. Only Adam and Eve were created, so no other people existed. For the first generation or two, brothers married sisters; this was not a genetic problem because the human race was still genetically pure, and God had not yet forbidden close relatives marrying.
Les explains that the common question of where Cain got his wife is answered by the fact that only Adam and Eve were created, so there were no other people. Therefore, brothers married sisters for the first generation or two. This was no problem because the human race was still genetically pure. In fact, even Abraham, 2,000 years after Adam, married a half-sister with God's blessing, and Isaac turned out fine. God did not put a taboo on marrying close relatives until sometime after Abraham, when the genetic pool was weakening and problems began to arise.