Your local library is an excellent source for humor. I found over 100 listings on humor in my small town library. They had tapes, records, books, and videos available and all at no charge. You can look for humor in the library under the following topics: Humor, Wit and Humor, Humorous Stories, Humorists, Humorous Poetry, Humorous Recitations, Humorous Photography, Proverbs, Caricatures and Cartoons, and Quotations.
Keep in mind that you can't always use the humor in these sources as is. You must update, personalize, and localize much of the humor to increase its effectiveness. Here's an example of a joke that came from 5600 Jokes for All Occasions. You will first see it in its original form from the book and then see the slight changes for the usable version.
Original - exactly as written:
Do you find that advertising brings quick results?
I should say it does. Why, only the other day we advertised for a night watchman, and that night the safe was robbed.
Update for presentation use:
Advertising can bring quick results. The other day my company advertised for a security guard and we got robbed that night.
To make the update for this old-fashioned joke, change the first line from a question to a statement to eliminate the dialogue between two people. Then change the old fashioned term night watchman to the more modern term security guard. Also eliminate some of the extra detail and say it was your company that did this so it's personalized. Now you have a quick usable two-liner that makes a point.