Boo Radley and Tom Robinson are good examples of "mockingbirds" in the novel. If you need more, then we have Jem Finch or Dill.
"Mockingbirds don't do one thing but make music for us to enjoy. They don't eat up people's gardens, don't nest in corncribs, they don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mockingbird." (10.7)
"Mockingbirds" in Harper Lee's To Kill a Mockingbird represent innocent people. Like mockingbirds which "don't do one thing but make music for us to enjoy", the "mockingbirds" in the novel are harmless, innocent people. Thus, killing or harming them is "a sin" because they don't hurt anyone. Tom Robinson is a very good example because he was judged and even killed due to a crime he didn't commit.
Hope this helps.