I've put these bells on all our bikes, attached just above the headset on the stem where it's easy to operate with a thumb, but out of the way:
http://www.universalcycles.com/shopp...97&category=70
I chose the Planet Bike version for the nice forged clamp, thumb operated dinger (easily rotated to the best position), and their donation policy. They are essentially same as the Crane version. The brass ones have a beautiful tone.