Although the Bible doesn't say where Mrs. Cain came from, Joseph Smith provided the answer in "The Inspired Version" of the Bible, which was his very own "translation". Here's what it says:
And it came to pass, that Cain took one of his brother's daughters to wife, and they loved Satan more than God. - Genesis 5:13
So according to Joseph Smith (and he ought to know!) Cain married one of his nieces. He doesn't say, though, which of Cain's brothers was Mrs. Cain's father.
Another answer is provided by the Pre-Adamite theory.