As a marketer, I am always looking for just the right photograph for an ad campaign. At times this may mean taking them myself, but very often, it's a matter of finding the right photograph in a stock photo company.
There are so many! Some like my favorite, istockphoto, are general warehouses of photographs on every subject. Others are very, very specialized. How magazine has an annual index of stock photo companies, and the last one was published in their July issue. If you use photographs a lot in your work, or want to, I'd suggest you go get it here. It's comprehensive, and all on one page.