Anybody looking for cheapish air horns, have a look here: http://www.priracing.com/section.php?xSec=107&jssCart=40262e3e06007d8b38e6e1f9adc6a4ea
I'm sure they're not perfect, but theyre a dam slight cheaper than anything else I've found, and they go down to 45mm....

Hope this is useful to someone!

Gregs