Where in the book does Jonas learn he can find his parents' parents names in The Giver?

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.

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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.