Where do I find the Finder Icon for ???-files?

I want to create a cool icon for my application, where can I find the icons the Finder uses for use as a template?
The Finder keeps most of its icons either in "/System/Library/CoreServices/SystemIcons.bundle/Contents/Resources/" (the really interesting stuff there is in "SystemIcons.rsrc", a data fork resource file), or in "/System/Library/CoreServices/Finder.app/Contents/Resources/". In these locations you should find most of the icons needed when creating a program's icons.

Note that Apple also have an official "Aqua Icon Kit" available on their web site that contains Photoshop templates for the most commonly needed icons. In particular, these are documents, plugins, preferences files and folders. You'll also find some "Actions" for Photoshop there which can be of great service.