Are there any directories of the official icons used in macOS?
Most of the icons are in the System folder. Just enter the System Folder, hit Cmd+F, click +, select Kind in the first dropdown box and Image in the second. This should get most of the icons.

Others are in the Apple Symbols font, as Tom pointed out.

A few more can be found here: /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources.