Where are Unity's UI Standard Texture files

For the life of me, one hour of Google search has turned up zip.

Where the f$%%$ are the standard images files for buttons, windows, etc? And when I set an UI’s Image to nothing (i.e. Panel), it appears to still use some sort of “standard” rectangle. Where the frell are these files!!! Not in Standard Assets folders either.

Thanks much.
Manny

Have a look here:
https://bitbucket.org/Unity-Technologies/ui/src/fd5d3578da8c33883b7c56dd2b2f4d4bbc87095b/UnityEditor.UI/UI/MenuOptions.cs?at=4.6&fileviewer=file-view-default

The pathes are :

        private const string kStandardSpritePath           = "UI/Skin/UISprite.psd";
        private const string kBackgroundSpriteResourcePath = "UI/Skin/Background.psd";
        private const string kInputFieldBackgroundPath     = "UI/Skin/InputFieldBackground.psd";
        private const string kKnobPath                     = "UI/Skin/Knob.psd";
        private const string kCheckmarkPath                = "UI/Skin/Checkmark.psd";

If you wanna load one of these into your UI Elements you can e.g. use:

    using UnityEngine.UI;
    using UnityEditor;            
    ....
    Image image =GetComponent<Image>();
    image.sprite = AssetDatabase.GetBuiltinExtraResource<Sprite>(kStandardSpritePath);

I don’t know about any possibility how to directly access the files directly e.g. in the Explorer.