Yes, they do make aftermarket horns, but the sound is a somewhat higher pitch, not quite the original sound. Before you order a replacement part, make sure you have a fresh battery I know that sounds too simple but that could be the case. Sometimes gently tapping the top of the horn several times with a screwdriver will loosen diaphragm and get it going again. If that all fails, you can unscrew the horn from the frame, and very slightly loosen and then tighten the set screw underneath the horn. Also, check the relay and make sure it is closing when you activate the horn lever and gently clean the contacts if necessary.
Hope any of these tips will help.
