Tracking down cartoons can be tough, unless you know exactly when they appeared and in which paper. One place to go for help is Daryl Cagle's Professional Cartoonist Index.
The Cartoon Bank offers searchable archive of cartoons published in The New Yorker. If you can't find what you're looking for, call the Cartoon Bank at 1-800-897-8666.
The University of Nebraska-Lincoln posts a digital Government Comics Collection.