Take a look at Producten (GB) | Valk Motive - All sorts. They have the Stebel nautilus and the Stebel magnums (just as loud but more conventional looking).
imo - bikes in WET/damp climates need marine grade horns. Durite Marine Trumpet Horn 0-642-14 0-642-15 - It's a DURITE item - which tends to mean unadventurous design but ROBUST. Loads of these on UK ebay too.