Google of course! Simply head on over to Google images at - http://www.google.com/imghp
Once you've loaded the webpage, type in the keywords for the images you're trying to find. Then once you've come onto a first page of results, click on the cogwheel to the right of search. Click on 'advanced search' from the drop-down box. you'll find yourself on this page... https://www.google.com/advanced_image_search?hl=en...
Now, where it says "type of image" click on clip art, and where it say licence change it to 'free to use or share, even commercially.
There you go! All legal for use! However I do recommend that you give attribution to the author of the image, by leaving a link to the original website.