'get info' for the documents folder (or any file/folder you like the icon of). at the top will be a small example of it's icon. if you click on this, you'll notice that's it's clickable: it gains a blue outline, it's highlighted. now press command+c, to copy it. edit will flash to confirm this.
do the same for the folder you want the icon on, but hit command+v instead of command+c, to paste it.
tada!