Cain knew his wife, and she conceived and bore Enoch. When he built a city, he called the name of the city after the name of his son, Enoch. - Genesis 4:17

"Where did Cain get his wife?"

Cain either married his sister or his niece. Genesis 5:4 says he had sisters, and the 130 years before Seth was plenty of time for Eve to give birth to Cain’s wife (Genesis 5:3). The reason people get confused about this is because they make the false assumption that Seth was Eve’s third child. The text certainly indicates that Seth was the third male child (Genesis 4:25), but we can’t force it to say that no daughters were born before him. This is especially true since the author mentions Adam’s daughters in Genesis 5:4 and assumes her existence in Genesis 4:17

"Why wouldn’t chapter 4 mention the birth of Cain’s sister/wife?" 

The birth of Cain’s sister isn’t mentioned because it’s not relevant to the story. The purpose of chapter 4 is to explain why Seth became heir in the lineage to Noah rather than Cain (Adam’s firstborn son). And since patriarchal lineages don’t name female descendants, we shouldn’t expect the author to diverge from the story and talk about her birth. To assume she didn’t exist because her birth isn’t mentioned is an argument from silence that ignores Genesis 4:17 and 5:4.

"Cain found his wife in the Land of Nod, so it couldn’t have been his sister."

This is an unjustified assumption. The text doesn’t say Cain found his wife in the Land of Nod, it simply says he "went away and settled" there. Then verse 17 says he had intercourse with her ("knew" her) and conceived Enoch (Genesis 4:17). However, this doesn’t tell us where he met her either. Even if he did marry his sister in Nod, she could have simply moved there before him. 

"But chapter 5 says all the ‘other sons and daughters’ were born after Seth."

Nothing in the text implies that all of Adam’s other children were born after Seth. In fact, it can’t mean this since Cain and Abel are lumped into the phrase “other sons and daughters", yet they were born before Seth. 

Chapter 5 isn’t a continuation of the story, it’s a genealogical record titled "The Book of the Generations of Adam", which records the lineage from Adam to Noah. It summarizes each male heir’s life, gives the name of their successor, and tells wether or not they had other sons and daughters. It doesn’t tell us wether these siblings were born before or after their successor; It simply records their existence. Attempts to place Cain’s sisters after Seth aren’t justified by the text, especially in light of Genesis 4:17. 

"Couldn’t Cain’s wife have been from a separately created tribe?"

No. The Bible is clear that the entire human race came from Adam (1 Corinthians 15:4). To posit separately created tribes is unjustified speculation that contradicts both Old and New Testaments.

"But incest is a sin according to the Leviticus 18:6 and 1 Corinthians."

Incest wasn’t forbidden until the arrival of the Mosaic Law, which wasn’t instituted until much later. And where there is no law, there is no transgression (Romans 4:15). God didn’t hold them accountable for the practical purpose of populating the earth from a single man. Instead, he commanded Adam and Eve to be fruitful and multiply. They certainly had direct commands from God such as not murdering, or eating of the tree of the knowledge of good and evil, but incest wasn’t one of them.