If you participate in activities related to your niche, why not take your own photos to use on your site?
Get in the habit of carrying a digital camera or camera-phone everywhere, so you can whip it out and get some original shots for your site.
NOTE: Before publishing your photos that have other people in them, it's a good idea to have these folks sign model release agreements, so they won't sue you later for using their image without permission. Many people are happy to let you use their image, and if they have a blog / website it's nice to link to it under their photo.
http://www.istockphoto.com/ - iStockPhoto is a fantastic source for nearly any image you could wish for. They also have videos and illustrations for sale. Their prices are reasonable and their quality is always very high.
Also peek into the "dollar bin" of discontinued images at iStockPhoto, and their free image of the week.
Be sure to read their terms and conditions to know how you may (and may not) use the content you buy from them.
These sites all provide free stock images. Some don't require anything from you in return; others request a link back to their site or just a credit to the photographer or artist wherever you publish the image.
Always check the “terms” or “about” page of free image providers so you'll know exactly how you may use the image, and whether anything is expected from you in return.
http://www.sxc.hu/
http://www.freefoto.com/index.jsp
http://www.morguefile.com/
http://www.dreamstime.com/
http://www.freestockphotos.com/
http://www.freeimages.co.uk/
http://photo.net/stock/
http://visipix.dynalias.com/index_hidden.htm
http://www.imageafter.com/
http://www.freemediagoo.com/
http://unprofound.com/index.php
http://www.niblack.org/imagebase/
http://aarinfreephoto.com/