I found some good teaching material.
These cartoons really keep the students talking.
I've had good luck with this book too.
Amazon.com: The Far Side ® Gallery: Books: Gary Larson
Any authentic material is good, and that can include cartoons.
The downside I've found to using humorous material with less advanced groups is that you often have to explain the joke... and then it isn't funny. Although, of course, the advantage of the Ferd'nand strips is that they're purely visual.