Who were Cain and Seth’s wives and where did they come from?
God probably made them too.
If you really want to know you can find the answer in Genesis 5:4.
They came from the land of Nod. You’ve read it.
The best answer I’ve heard is, by the time Adam was a thousand years old, he would have fathered about 800 children, who, over a hundred years here and a hundred years there, married and had their own children and great-grandchildren by the time Cain was ready to marry. So, Cain married one of his 100-yr old brother or sister’s great-or great-great-grandchildren. Because Adam lived so long and Eve also able to crank out the kids for those same 800yrs, we can reason that The Almighty kept Adam and Eve’s DNA incorruptible, so there was no initial danger of inbreeding or disease. Once there were thousands and thousands of people far removed from Adam, The Almighty declared incest a mortal sin and DNA became corruptible.