There are lots of sites out there that offer free icon sets. You can get icon sets for social networks, for themes, for badges, there's even an icon finder
you can use to find the perfect icon you need. But I've often been perplexed by this. I suppose it's because I tend to build my own graphics, but I have never seen why someone would want to use an icon for anything but tiny things (like a Facebook link or something). And I'm not really sure what the difference is between an icon that's not in ICO format and a graphic. Is there one?