A pair of these might work.... http://www.aerostich.com/fiamm-freeway-blaster-horn.html
No personal experience with them.
I have this setup and I like it!(bought mine at a local auto parts store)
It's not Plug-N-Play as most dual or louder aftermarket horn setups aren't.
I used a relay which is activated by the horn wire to supply direct power from the battery.
It's a tight fit to locate them where the factory horn is (that's where mine are located), but it's doable.
I've helped others with the installation of dual horns at wrenching sessions, so if/when you make your decision on which horn(s) you want, find a COG Wrenching Session in your area for assistance with the installation.
