Pay attention to Ebay. Might have some luck on facebook marketplace for calls as well. I dont play on any social media sites though. Did away with that many many years ago. I see Kingyon calls come up a few times a month. They certainly arent the most popular anymore and collectors have pretty much gobbled them up. Keep your nose down, keep hunting, and you'll stumble on one. I always say it, just have your pocketbook ready when one comes up, they dont come up for sale often and when they do, usually want a pretty penny for one anymore. Its a funny thing, you will look for a call for months and months and then a collector will start to move some inventory and you might find 6 in a week. Just how it goes. I looked for my RNT OSDC collection for about 9 months and then in 3 weeks I put them all together. So just stay persistent and stay patient and you will find it.
I would never pay $700 for a kingyon, but I would pay $250 for one, especially a duck call. Goose calls are far less valuable than a duck call. Just the way the market is, and I contribute that to being able to manipulate goose calls far easier than duck calls. One of my fav goose calls has 3 different companies to make it up. Can't do that with a duck call. The toneboard is cut how the toneboard is going to be, and thats why I think they hold value so much more than goose calls.
As a collector myself, I am always looking. Kingyons are not a high priority of mine. If I stumble across one though, I will send a link your way.