Get a la cucha racha ' horn ! They'll never expect that!
That way you'll be able to tell yours from everybody elses!
Oh well, just a thought.
Actually just about any 12v alarm siren will work. Here's one from Commando:
It's about $10 from
Custom Street Designs
But there's probably someone in your town that stocks them too. Try Circuit City or one of the stereo shops.
-Mike