Didn't they come from a nearby city?
In Genesis, there's no explicit mention that Abel had a wife. But after Cain killed Abel, Cain fled to the land of Nod, east of Eden. Chapter 4, verse 17 reads, "And Cain knew his wife, and she gave birth to Enoch." Cain also was the founder of the first city, also called Enoch.
Genesis explicitly states that Adam and Eve conceived Cain and Abel at the beginning of chapter four, but doesn't state where Cain's wife came from. There's basically two theories:
1) Adam and Eve also had other sons and daughters not explicitly mentioned, and Cain's wife was actually one of his sisters who went with him into exile. Yes, it's incest, and no, inerrantists don't have a problem with that because reasons.
2) Jehovah created other people besides Adam and Eve but the author of Genesis didn't mention it because reasons. You may be forgiven for being led to believe that Adam and Eve were the only people specially created by Jehovah, but that's just because you haven't studied Hebrew. Much is made of the fact that in Genesis 1:27, it reads "God made
mankind in his own image," not "God made
a man in his own image." There's plenty of other instances where "mankind" means multiple people, so perhaps Jehovah created thousands of people, picked two to be his special pets, then got furious when they wouldn't follow orders. Later, Cain merely found one of these other unmentioned women and made her his wife.
Both arguments have adherents and their problems.