Fortunately the Internet is full of great resources, and thanks to technology it’s all searchable.
Resource #1 is Google Images. Type in what you need, hit search, and then browse thousands of results. However, be cautious in reuse of these images, especially for commercial purposes. Most of these are likely to be copyrighted. If they were obtained from the stock photo vendors, like Corbis or Getty, they were purchased at no small cost. Likewise if your competitor paid for a photo shoot they probably would not approve of you “borrowing” what they spent good money on. However they can help you locate appropriate images, and in many cases you can ask and obtain permission to reuse.
Corbis and Getty aren’t the only sources for stock photography. Resource #2, istockphoto.com, is an excellent source for inexpensive stock photography. Small low-resolution images, appropriate for most web needs, cost little more than $1. Freelancers world-wide contribute their work so the selection is vast and the quality has improved dramatically over the last several years. Their images are royalty-free which means once you purchase it, you have rights to use it as you see fit.
The third resource is Flickr. Most people think of this site as an online shareable photo album. However, many choose to allow others to use their photos via the Creative Commons license. This reserves some rights but allows a wide variety of usages up to including commercial use and creation of derivative works.