Just in case this is news to anyone...
I'm sure most people are already aware of this, but I thought it would be best to share this for those of you who might find it useful:
This collection of images are free (most of which are free even for commercial use) without the need for author attribution. There are also some pretty useful editing tools on here for those of us that don't have access to Adobe Illustrator...
While some of the images can be fairly basic, there are some real gems on here if you're prepared to search through.
Anyway, happy searching if you've not seen this before!