Where did Cain get his wife

In the rules of Rabbi Eliezer:

Extension: The particles “et” את, “gam”, and “af”, which are superfluous indicate that something which is not explicitly stated must be regarded as included in the passage under consideration, or that some teaching is implied thereby.

This rule is used by Rashi to derive from the Hebrew that Cain had a twin sister and Abel has two twin sisters (triplets). Genesis Rabbah 22:2.

So there is manuscript evidence that there were more than just Cain and Abel.

והאדם ידע את חוה אשתו ותהר ותלד את קין ותאמר קניתי איש את יהוה Ge 4:1

ותסף ללדת את אחיו את הבל ויהי הבל רעה צאן וקין היה עבד אדמה Ge 4:2