Check the wiring on the car- the horn circuitry (especially the switch, an expensive repair, usually, with airbags in the steering column) may not be able to handle a louder horn without adding a relay, and that relay may deliver enough more voltage to the existing horn to satisfy.
Otherwise, shop around; Fiamm makes some great and loud after-market horns that are quite compact. But try the relay first, you’ll want it for the replacement, anyway.
the horn voltage doesn’t go thru the steering column on most modern cars - they use a relay setup and only popping the relay on and off with the horn switch… so, you should be safe adding most automotive-variety horns to any car.
J.C. Whitney used to sell a locomotive-sounding horn for cars years ago, but I haven’t seen one lately. Seems they pulled it off the shelves, not because it was too loud, but it scared the drivers around the “honker” thinking they were about to get hit by a train!
The caller should first compare the horn’s sound against another vehicle of the same model. Maybe the car’s horn hasn’t been working right since day one. Most cars have two horns (high and low pitch), and perhaps one has never worked right. He might be satisfied just reconnecting (or replacing, if defective) the existing horns.
If he’s unhappy with “normal” horn sound, the answer may lie with a shrink, not an auto parts store. It sounds like he may have anger or dominance issues, and an obnoxiously loud horn isn’t going to be the end of it. He should work on not having to pound the horn button at every minor provocation.
By the way, this thread should have been started in “Second Opinions”.