There are plenty of stock photography sites out there, and while many of them are expensive, more and more of them offer some great, affordable images to use in your designs. But before you decide on your image, be sure to read through this post at DesignShack titled 10 pitfalls to avoid when using stock photos.
The article covers topics such as avoiding cliché and overused photos, creating bad clipping paths, and avoiding obviously posed photos.
Perhaps the most important piece of advice offered is to shoot the images yourself. A decent DSLR camera can be had for $500 nowadays, and can produce stunning images in the right hands. Beef-up on your photography skills by reading through the great photography tips and reviews at Digital Photography School, or subscribe to a photography podcast such as Daily Photo Tips with Chris.
Regardless if you choose to buy stock photos or shoot them yourself, be sure to make good choices when selecting images for your designs.