Where or when in To Kill a Mockingbird can I find a quote that talks about Atticus having to accept to defend Tom Robinson because he must teach Scout courage?
By taking the challenge of defending Tom Robinson, Atticus teaches the importance of courage to Scout.
Atticus does not take on the defense of Tom Robinson specifically so he can teach Scout about courage. Atticus was requested by Judge John Taylor to defend Tom instead of the regular public defender. Taylor must have known that Atticus would give Tom the best defense possible, and Atticus reluctantly accepted.
"You know, I'd hoped to get through life without a case of this kind, but John Taylor pointed at me and said 'You're It.' " (Chapter 9)
He must have realized that defending a black man accused of raping a white woman would be unpopular with many people in the town, and that it might bring trouble to him and his family. Atticus tells his brother, Jack, that he had little choice.
"But do you think I could face my children otherwise... I hope and pray I can get Jem and Scout through it without bitterness... I just hope that Jem and Scout come to me for their answers instead of listening to the town. I hope they trust me enough." (Chapter 9)
Atticus's humble nature precluded him from claiming that he did something himself in order to teach Jem and Scout about bravery, and I'm sure that Atticus didn't consider his defense of Tom as being courageous. He did express to Jem in Chapter 11 that, following Mrs. Dubose's death,
"I wanted you to see something about her--I wanted you to see what real courage is, instead of getting the idea that courage is a man with a gun in his hand."
This quote indirectly refers to Atticus's killing of the mad dog, and it is an example of how inner courage is even more important than physical bravery.