Since it is present when you add a new baseboard then it would have to be built-in, so you do not need to download it.
The easiest way to identify it would be, in:-
Surveyor Classic (S10): open the Texture tools, click the Get texture tool icon and move the mouse pointer onto the texture. Its name should appear in a tooltip box under the pointer.
Surveyor 2.0 (S20) select the Eyedropper Tool and move the mouse pointer onto the texture. Its name should appear in a tooltip box under the pointer.