You may have to go to the dealer or junkyard for that one. Depending on how big it is, however, you might be able to find something comparable in one of those "Help" kits in the auto parts store. I was able to replace a missing rubber bumper from behind the license plate with a bumper from one of those kits (not really a kit--just a bunch of various small rubber bumpers), and the whole kit cost less than half the cost of the single OEM part.
2005 Dodge Grand Caravan SXT, 129K mi.
Former owner of:
1988 Dodge Caravan SE, 2.5L I4 (1988-1995, 99K mi); 1995 Plymouth Grand Voyager SE "Rallye", 3.3L (1995-2004, 169K mi); 2003 Grand Caravan Sport, 3.8L, (2004-2008, 106K mi), 2008 Nissan Quest, (2008-2010, 25K mi)
2008 Lexus RX400h (Dark Gray), 112K mi; 2005 Hyundai Elantra GT (Blue), 89K mi