Where can I find a list of cartoon that are "public domain" (copyright expired)?

Below is a link to information regarding how you can determine whether or not a copyrighted work is in the public domain.

http://www.copyright.cornell.edu/public_domain/

For example, cartoons published before 1923 are in the public domain. Further, cartoons published between 1923 and 1977 without a copyright notice are also in the public domain. Please see the link above for other examples.

I doubt there is a singular source showing a list of all cartoons (at least with any confirmed accuracy) that are in the public domain. Most likely you will need to do an independent analysis with each cartoon you come across.

Edit: While Simon's link is a helpful starting point, I noticed some of the information on there is incorrect. If you do rely on it, make sure you do your own independent analysis.

Eureka! (my state's motto). After around 15 minutes of searching different terms on Google I found it. Thus, You've Just Saved $200 + a Tip:

http://answers.google.com/answers/threadview?id=357341

Google used to host an answers forum of their own except people paid money to get their questions answered. The answer is near the bottom. He's listed a bunch of websites that should have the infomation you are looking for.