Originally Posted by
10 Wheels
$7 a Loud Ding Dong

I would say that
is probably the loudest bell you can get. If you're ok with the "ding-dong" sound. If you want a more tradition sound, then there are these:
http://www.amazon.com/Classic-Retro-.../dp/B000OF7JEQ not as loud, but pretty good sized. But you cannot mount them very well on drop bars, or on the handlebar stem.