Here are a few loud horns:
The hornit: http://www.thehornit.com/
A shop I used to work at sold these and they are extremely loud - around 140 decibels, but they sound like a loud beep rather than a horn. It takes 2 AAA batteries. Costs $45 US. The nice thing about this one is the button to press is remote, so you can have the horn on your fork or wherever.
This one is slightly less loud (115 db), but it has more of a honk to it and requires no batteries - it takes air pressure through your standard bike pump, so there's virtually no cost beyond the initial investment. this one is slightly bulkier as you have the horn itself, the air cannister, which looks like a water bottle, and the tube that connects them. A friend of mine has one and he says it's pretty effective.
There's also this kickstarter: https://www.kickstarter.com/projects/lansey/loud-bicycle-car-horns-for-cyclists
I've never seen this one in person, but the kickstarter can explain it for you. It has 112 db sound and sound the most like a car horn. it's $95 US though.
GaryE also showed this one:
I use a bike horm from another Kickstarter project. It has a friendly sound and a very loud
beep: ORP Smart Horn – Gary E
The loudness is 98 db, but this one also has a light included