A great source of gags for single-panel cartoons is song, book, movie, television and Broadway show titles. If you pick a familiar title (current or in the past, although current is better) and change it or do the reverse of it, you can many times come up with an idea for a cartoon.
My local library loans movies and I've been known to go down the rows of movies, with pen and paper in hand, and jot down titles that I think would lend itself to a great gag. Same for what seems to be thousands of books in my house, many from my college days as an English major. These titles, well known because they're classics, can also be used, as can song, television and Broadway show titles. Try doing the reverse of the title and see if it becomes funny. Then visualize how it can be illustrated.
Of course for those who use the Internet for all kinds of information, you can find titles of almost anything online. Just Googling "Top songs of the '60's" will get you all kinds of good stuff. (It will also bring back all kinds of memories, hopefully good ones. You might spend so much time singing to the oldies that you'll forget about the gagwriting).
Good luck with these ideas and let me know if they help you. You can send me an e-mail at: email@example.com.