.icns

Here's one place to download all kinds of free icons.

http://iconfactory.com/home

Scratch that, it used to be a good place to find free icons. Try here:

http://interfacelift.com/icons/downloads/date/any/

http://findicons.com/search/mac

At this last one, you're not limited to the Mac icons on the page, it's just showing you icons tagged with "Mac" as a keyword. Put anything you want into the search bar and download the .ico version of anything you like.