Who buys Scout and Jem air rifles?

The air rifles open up the first conversation about mocking birds and how it is a sin to kill them because all they do is make sweet beautiful music. You later learn that Tom Robinson is being represented kind of like a mockingbird, he had done no harm, but they wanted to kill him anyway. Had they not gotten air rilfles you would have been clueless to where the title came from, and it's to big a part of the story to miss.