I have a client that will require me to have at least 20-30 page header type images/photos, probably on a rotating basis. I suspect they will also want to change the photos from time to time. So, I need to have some business photos, construction, manufacturing, legal, etc. and a fair amount of them.

Can anybody recommend a resource to find and legally download such photos? Free would be great, paying for a collection would be fine, or paying on a per photo basis would be fine as well.