Where are the default system icons for folders and files stored?
The default icons are stored in various locations inside /System/Library - is there a specific icon you're looking for?

Finder.app (/System/Library/CoreServices/Finder.app/Contents/Resources) has the Smart Folder icons and the Burnable folder icons for example.

Most of the more known icons (eg. pictures for the default folders among many others) can be found at /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources