73 Challenger horn relay - where to buy from

The relay for the horns on a 73 Challenger is located on the fusebox, correct? My car has never had one. Where can you purchase one of these? Does it also function as a buzzer? I found this part number 3738109 on this list:

http://www.cuda-challenger.com/cc/index.php?topic=37217.0

Is this the correct one? I checked Rock Auto and could not find anything.

I got my horn relay for my 70 RR at NAPA..RONNIE