Query: "Zzz" for Snoring?

A friend forwarded this query to me to pass along. If you have any suggestions, please send them directly to James Harbeck via his email address.
Date: Sat, 19 Feb 2011 22:44:58 -0500
From: James Harbeck
Subject: zzz?

If I were to want to trace the history of the use of ZZZ for snoring in cartoons, what would be my best way of doing that? I have to assume that there's something better than dumpster-diving old comic books (wherever they even may be found), but I'm not entirely sure where to start looking.

Thanks!

James Harbeck