Where can I find the textures from the default GUI Skin?

I created a new GUI Skin for my game and I am using it to display some nice Labels, Text Areas, etc. The problem is that I accidentally selected one of my existing textures for the Box -> Normal -> Background in the GUI Skin. And now, the default nice semi-transparent black "box" texture that was there before is nowhere to be found.

The same is true about any texture that comes with the default GUI Skin (e.g. textfield, textfield hover, textfield on, etc.) If you select None or any other texture for these, than the defualt texture that was there is gone forever (unless you create a new GUI Skin and do all your other settings all over again).

Are those default textures part of the Assets that come packaged with Unity? And if yes, where can I find them or how can I get access to them again if I deselected them in my GUI Skin?

The textures are available in this thread on the forum. There's both the Photoshop source as well as a UnityPackage available for download there.

It’s now on the asset store Built-in GUI Skin | 2D GUI | Unity Asset Store

You could also restore them programatically by creating a new temp skin then copying things across from it with some temporary code. E.g.:

GUI.skin.GetStyle("button").normal.background = guiNewTempSkin.GetStyle("button").normal.background;