I just had somebody asking where they could find artwork for all the standard signs. A book from the US government immediately came to mind. The Standard Highway Signs gives you the specifications for all of the standard signs.
Now you might wonder how the book provides the artwork. It is supplied as an unlocked PDF file. That means you can bring any of the pages into CorelDRAW for editing. It may take a short while longer than just clip art, it doesn’t take much more and gives you all of the information you need directly from the government.