Pretty scary. Glad to hear your OK! I don't know of any personally. Like the NYC bicycle messengers, I know a whistle really gets a drivers attention. (I know because I almost backed into one struggling up a hill) Cool part about their way, inmho, is it leaves your hands free.
These folks have really cool old world bike horns; Parts and Accessories - Rideable Bicycle Replicas
They are huge and surprisingly loud, but don't think loud enough to be heard in a car.
Google came up with this, looks like it would work for our needs, Ultra Loud Bicycle Air Horn - Truck Siren Sound (120dB) [E18132] : BestOfferBuy.com, Buy DVD, Shop for PC accessories, Discount MP3 Players, Bargain Deal for Surveillance Equipment, Cheap R4 for NDS, X-sim Unlock