These two sites have routinely saved my hide.
Brands of the World has just about any logo you would ever want, in a true EPS format.
The Stock.Xchang has thousands of truly free stock photos. Make sure to check the rights usage for each photo, though.
Another that I’ve used is LogoTypes. You can always get the major big corporate logos and some smaller companies too.
I collected my favorite vector based resources and some that where new to me in a post on my blog at:
GeneralSpecialist.com