Note, all the suggestions below are 'direct' replacements for your stock horn. They don't necessarily require a relay or any aftermarket brackets. If you plan to install an air horn it is strongly suggested you use a relay to prevent damage to your horn switch. You'll find instructions for that type of installation this Fixit page.
Idea One From Earl Riviere (TYNKER)
Here is a $10.00 trick for you. Go to a AUTO ZONE store and buy an auto replacement horn (they look like a rams horn) high pitch Curves to left, low pitch curves to right. Remove your right side horn,and you will see how you need to bend the supplied bracket to fit into stock mount (it will clear cylinder head and radiator). If you want more sound, do the same thing with left side. It is MUCH LOUDER with one side stock horn and one car horn. Try it,for 10 bucks you cant go wrong.
Idea Two From xonepercent
Went to the local junkyard and got two horns off a Ford Taurus, $5. Now I sound like one of the enemy, they are loud.
I'm using a Howard's Big Air Horn. I did have it mounted where the stock ignition switch was but didn't like the way it looked...so i moved it to the right rear. Compressor is located where the Goats Bladder use to be. Very loud.
Link to Accessories International "horn" page. Nice assortment
Idea Five From Lectricman
This may be the same as #1 but I bought a set of FIAM 125 db horns at the NAPA store for $15 each. I took off the factory horns and mounted the FIAMs from the mountig points for the factory horns with the brackets that come with them. They mount just above the crash bar tucked in by the down tubes. I painted the backs black and think they are unobtrusive hidden as they are by the fork tubes.There is a high and low pitch horn in a set of 125db each. Really gets attention! The horn leads reach also. (These horns are available on Amazon also.)
Before installing the Stebel Air Horn I used the same FIAMM horns described above by Lectricman (one high and one low pitched) but installed them 'behind' the radiator, pretty much where the stock horns were. The left side installs very easily using the stock bracket. The right side requires a bit of bending and fiddling with the location but it 'does' fit without interference with the engine or radiator.
Gadget note: Be certain you buy a replacement horn with two contact posts (unless you're going with air horns). Single post horns are designed to self ground and won't work for most motorcycles which always have power applied to the horn circuit. Pressing the horn button just completes the path to ground.
Have another solution? Let Gadget know so it can be posted here.