I thought you were asking where the names were found, but maybe you are asking where in the book this is mentioned? I have changed the question for you. The Hall of Open Records is first mentioned is in chapter 2, on page 17. The names are in the Hall of Open Records and anyone can look at them. Jonas considers that he can look up the name of his parents’ parents in chapter 16, on page 124, in this conversation with The Giver.
Jonas frowned. “But my parents must have had parents! I never thought about it before. Who are my parents-of-the-parents? Who will be their grandparents?”
“You could go look in the Hall of Open Records. You’d find the names. But think, son. If you apply for children, who will be their parents-of-the-parents? Who will be their grandparents?”
“My mother and father of course.”
Jonas learns that the concept of grandparents has no meaning in his society, because families only stay intact long enough to raise the children.