try ebay

The following is a listing for all 12v horns by lowest price.
Ps you don't need an expensive horn for it to be LOUD. Mine were 10bucks and that included shipping for a (dual) set of 130dB loud!! Fiamms.

http://search.ebay.com/12-volt-horn_W0QQfromZR40QQfsooZ1QQfsopZ3


Gooday
Jim

small wheel moves by fire and rod,
big wheel fires by the grace of god,
everytime that wheel turns round,
bound to cover just a little more ground.