Go to a junk yard and get one or more horns. Power them with a relay, and you'll be set.
If you want to go big, go here: Train Horn Kits & Air Suspension By HornBlasters, The Name In Locomotive Air Horns!
For a bit less money, you can find one here:
super loud car horn | eBay
In high school I had a '59 Oldsmobile. There was a giant space between the radiator and the grill. I went to a junk yard and got about 8 horns and installed them in that space. Worked great until the relay contacts welded shut while downtown and it was a race between me disconnecting the battery and the constables arriving. I won.