u sure can get aftermarket horns.
u can get ur standard horn like most cars have, but u can get louder ones..
but if u want something loud. go air horns.
u can get from 1 to about 6 horns
if u are going to replace ur normal horn there east to install as the compreser hooks up to the wires that are already going to ur cars horns.. then u just gota mount the horns and plum some plumbing. easy and loud.