Everyone (well, a very large number of people) uses iStockPhoto for stock imagery.
It’s a great source because they have millions of photos, you know you’ll be able to use them without worrying about copyright issues, and they are inexpensive.
However, they can be spotty. For example it is hard to find good photos of anyone over about 40.
There are other stock photo sources that can fill in some blanks, though if they have anything iStockPhoto doesn’t, they are probably a lot more expensive.
Of course very specialized photos you aren’t going to find from stock sources. Like before-and-after photos of dental work.
On the flip side, there are some usable sources for free photos.
For example, NASA has a large stock of photography which they specify you can use.
Here’s a sample photo and their usage notice:

Discovery Blasting Off