That is one of the main (and supposedly unanswerable) questions that many unbelievers or skeptics use in order to try to "trip up" believers. But, God doesn't make mistakes and the answer CAN be found in the Bible.

In a nutshell, the answer to the question as to where Cain got his wife is:

• The DNA was pure in those early years, untainted.
  
  
• The Bible doesn't say that Hava had any children before Cain; however, in Gen. 3:16 ADONAI told her He would "greatly increase your pain in childbirth" which suggests she MIGHT have had children by then. Because the DNA was pure in those early years, YHWH did not prohibit marriage between close relatives until the time of Moses (see Leviticus 18-20). Based on Genesis 1 and 2, marriage was a one-man, one-woman partnership for life, which meant there was no disobedience to His law in the beginning when close relatives (even brothers and sisters) married. YHWH blessed the union between Abraham and his half-sister (Genesis 20:12) which produced the Hebrew people through Isaac and Jacob. That was approximately 400 years BEFORE He gave Moses the laws to prohibit such marriages.
  
  
• Most believers find it difficult to answer the question about Cain's wife because they see this dilemma through today's myopic eyes, using their limited human mindsets to try to understand the clear historical record that YHWH has provided.

We recommend you visit ChristianAnswers.Net, whose writers have done the best study we have ever seen on this subject in an indepth article called, Where Did Cain Get His Wife?