What other works by Vonnegut, reviews, critism or other secondary sources would be good to support the message that Vonnegut is trying to convey in Harrison Bergeron?
Vonnegut felt that the medium of television had become rather detrimental as documented in one of the essays on enotes to which the previous poster refers you. Therefore, you may wish to access his words on the medium of television with its instantaneous information, and its desensitizing and thought-shifing properties. Arguments that Affirmative Action and the No Child Left Behind law advocate mediocrity and uniformity in people can also be supported in "Harrison Bergeron."
You might want to follow the link below:
This will take you to a range of essays based on this excellent short story looking at it critically and the kind of message that Vonnegut was trying to convey. You will find essays on how the story can be viewed as a satire and what Vonnegut is trying to communicate about society through this tale. Hope this helps!