Hey mtkawboy, I would also add the relay like broncoman said. I don't think your '69 had one from the factory, so all the juice goes through the horn button and causes a buildup of, I guess oxidation would be the best term, on the contacts. Even good horns won't sound too good when this happens because it reduces the available power to the horn. My '71 lasted for about ten years the first time, but after that I had to clean the contacts about once every 6 months to keep the horn working properly.
Until I added a relay that is. Now I never have to mess with it. Horns take a surprising amount of current to work well, so give them a fighting chance. Not saying you don't still need a new horn, but the button sure takes a beating.
Good luck, and like the rest were saying, just about any O.E. horn uses that same blade connector so you should not have too much trouble finding one. Some decently loud ones I've heard are post '88 Chevy trucks, and just about any Cadillac. But the Caddy's are pretty unique so your Bronco would sound just like a Cadillac if you used them. That would raise some eyebrows on the trail. Hah! As for Fords, I would think the Lincolns and Crown Vics would work. The newer trucks might be decent too, but don't go for older ones. My '79 had horns that were wimpier even than the Bronco's!
Paul