If Adam and Eve were the first humans on Earth, where did Cain and Seth get wives to have their descendants?

There is a difference in the Hebrew that you can see in a Green's Interlinear, between adam, which means in general, versus eth ha Adam, which means, The Man Named Adam.  This matters because it was the daughters of ha Adam, specifically, that the fallen angels attacked in Genesis 6 to try to prevent Jesus from being born into that lineage without fallen angel blood, as God had predicted "her Seed", meaning Jesus, would bruise satan's head in Genesis 3:15 KJV.

If you read Genesis 1 and Genesis 2, you will see that God made many people on the 6th day, then He rested on the 7th day, then He realized He had not made a farmer, someone to till the soil, then He made eth ha Adam on the 8th day.  In addition to making eth ha Adam from the clay, like the 6th day people, God breathed His Word into eth ha Adam, which means Adam understood God's will in His Word. Now, when people choose to study and pray that God guides them, He breathes His Word into them and they become living souls.  Jesus Christ is also known as God's Word, John 1:1, John 1:14 KJV. Digesting The Bread of Life, means digesting God's Word, food for the soul and mind, chewing it over, pondering on it all, questioning, and finding answers from God in His Word. 

God's Word is written in the stars and this is how His Word was taught and handed down prior to Moses' scribe work.  God calls this the 'mazzaroth' in Job 38:32 KJV.  Ever wonder why so many diverse civilizations grouped together and named the same constellations?  The tower of Babel, 'its top with the heavens' means that God's Word in the stars was painted on the ceiling, like in the temples of Denderah and Esneh.  Some people were trying to keep God's Word secretive in order to try to control other people, which was never God's will. You can learn more about God's Word in the stars, the 'mazzaroth', here https://levendwater.org/books/witness/frameset.htm... 

When Cain left, he went east and took a wife from among the 6th day peoples.  Cain knew God's Word in the stars, but did not understand God's will.  Cain lived a long time and falsely taught God's Word, due to his misunderstanding, to many people.

Lilith is not in The Bible.

Genesis 1:2 KJV And on the seventh day God ended his work which he had made; and he rested on the seventh day from all his work which he had made. 3 And God blessed the seventh day, and sanctified it: because that in it he had rested from all his work which God created and made. 4 These [are] the generations of the heavens and of the earth when they were created, in the day that the LORD God made the earth and the heavens, *5 And every plant of the field before it was in the earth, and every herb of the field before it grew: for the LORD God had not caused it to rain upon the earth, and [there was] not a man to till the ground.* 6 But there went up a mist from the earth, and watered the whole face of the ground. 7 And the LORD God formed man [of] the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul. 

Ezekiel 18:23 KJV Have I any pleasure at all that the wicked should die? saith the Lord GOD: [and] not that he should return from his ways, and live? 

2 Peter 3:9 KJV The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance.  

https://www.youtube.com/watch?v=riJyJ0wvcM0 

That assumes that all of the created humans were recorded. Perhaps there were others not mentioned.

Haven't you ever read the Holy Bible? Or perhaps you've had a false teaching from some heretical religion?

Genesis 1: 27-28. God created man in his image; he created it in the image of God; he created them male and female. 28 God blessed them; and God said to them, "Be fruitful and multiply; fill the earth, make it subject, dominate over the fish of the sea and the birds of the sky and over every animal that moves on earth ».

Verse 28, God says, be fruitful and fill the earth? How do you fill the earth? Only of Men?

It is clear that, Eve, in being fruitful, he also gave birth to women, because being fruitful means giving birth both sons and daughters.

Don't read too many comics, Lilith is a devil character, demons are asexual!

If you have problems knowing other things, enrol in the Protestant Bible school, you will be given the necessary scriptural, and verbal teaching!

You can put your shirt on now, or you'll get the flu!

There were over 5 millions people on earth when God sent Adam and Eve (Nephilim Sons of God) to preach to, not mate with mankind(forbidden fruit). If you mate with them, you die(lose supernatural power). It is GOSPEL that makes man in God's own image. Adam was made fully aware of the consequence of mating with humans: Eve was NOT.

@6000 years ago, when Eve mated with a Nodite tribesman from east of Eden, Cain was conceived. Cain was not Adam's son. Then Adam mated with a Nodite woman.

It was Abel's mocking Cain as a stepchild that provoked Cain to retaliate.

The Babylonians forced the Jews to rewrite their history to reflect that all humans come from Adam and Eve, not just the Jews.

Science and religion agree (again). The bible speaks of other humans. Nephilim are said to be large (perhaps Neanderthal, which we know, now, bred with modern men (not including Africans)). Neanderthals had blond or red hair and blue eyes. Ezekiel 32:27 says that "when people began to multiply on the face of the ground" the Nephilim were fair. Ezekiel described Adam's sons as "sons of God." Ezekiel mentioned that these "sons of God" bore children with the daughters of humans (could these be the Neanderthals?). The word Nephilim means giants (possibly Neanderthal). Numbers 13.33 says that the Nephilim later inhabited Canaan at the time of the Israelite conquest of Canaan, and that they were strong, and very large. The Haaretz website says that teeth found in Manot Cave, in Galilee likely belonged to humans, 40,000 years ago, who interbreed with Neanderthals.

First does not mean only.

You really shouldn't take this so seriously!

The Bible is just a collection of very unrealistic, silly myths and legends.

None of those people really existed.

What you're asking is almost like asking why Red Riding Hood couldn't immediately tell a wolf from her own grandmother!

Adam and Eve had many sons and daughters.Cain married one of his sisters like his other brothers did.Eve likely gave birth to many children before Abel’s murder.  

First, separate myth and fantasy and parable from each other.

You are assuming that they were the only humans on earth at the time but there is evidence that there were already plenty of other people. When God confronted Cain and cursed him for killing Able Cain replied.

14 Behold, thou hast driven me out this day from the face of the earth; and from thy face shall I be hid; and I shall be a fugitive and a vagabond in the earth; and it shall come to pass, that every one that findeth me shall slay me. Genesis Chapter 4.

Who do you think Cain was talking about?