I would search for stock photography sites. Am I allowed to use the Goo* word here? When I used THAT search engine just now, I got hundreds of hits on sites like Shutterstock or Getty Images. You get what you pay for in this case and professional sites give professional results.
Tip #1: Look for Royalty Free images. They're not free, but you can use them again and again.
Tip #2: Open any MS Office product and say you want to Insert > Clip Art. Then tell it to look for "Clips Online". Microsoft has put together a pretty good library of free stuff that you can search.