Tough to find in any condition, "working" condition almost impossible.
If you have the correct originals NOT working, they can be rebuilt. Be sure you know the correct orientation of the rear bracket before buying or restoring any of these. Correct LEFT Side horn will point straight downwards and mount with the AC Condensor, different than a NON-Air Conditioning car. I'm thinking the RIGHT horn is same AC or No AC. Don't know at this moment which is "Hi" or which is "Low" note.
I do not have a good picture handy of the correct ones but will look to see if I can find a picture. (Check back) Here are a couple example shots that are not to clear, maybe a bit of photo shop enhancement can help.
It has been discussed in other threads and pictures have been posted of them too.
NOTE: Both images posted are of RESTORED cars. Other details noticed may NOT be correct for your example. For example, the AMBER Fog Lamps on the Green car, those are not "Concours Correct".