I went to the Apple Store when I realized I must have left the cord to my wifes iMac behind after a move. They guys at the Genius Bar just gave me one. Granted it wasn't the best looking cord in the world as it had gotten pretty dirty, but a cord is a cord and it worked just fine.
The connector is a standard IEC C13/C14 pairing, so any generic cord with that plug will work, but it won't have the decorative disc that Apple uses to conceal the connection.
If you're concerned with the cosmetics, you'll need to obtain an Apple cord.