It needs to have a Peak Reverse Voltage rating above the relay's coil voltage, and a Forward Current rating greater than the coil current.
Since I am new to this where do you guys prefer to order your parts? I'd like somewhere in the U.S.A. that is priced reasonably with fast shipping and good customer service. Of course I'd pay slightly more providing the last two criteria were exceptional.
NC o o NO => to horn, other wire to ground (only triggered by ard) \ o <==12V (common)(Ard) coil o====o coil (ground) --|<|-- (diode)