PC3 breath controller
I've looked at these before.
They certainly claim to be 100% compatible with the Yamaha BC line and therefore in theory should work perfectly with Kurzweil's compatible keyboards.
I haven't tried one myself but do get back to us if it works for you. I might pick one up myself. Kinda pricey but what isn't these days?
I've decided to take the plunge and order one from Ebay.
It should get here in another week or two. It seems to be coming via snail post from Sweden.
I'll post my first impressions here once it arrives.