Tirerack claims they can shave new tires they ship to you. That’s one way to buy just one tire.
Buying a used tire of the same size, sounds like a good option but there are problems. If it is a used Kelly tire of the same model and size - fine. But that isn’t likely. If you find a used tire of another brand with similar tread depth it still has different sidewalls and specs so that due to tire flex and such it will not have the same rolling circumference. In other words it might be different enough to damage an expensive to repair AWD system.
Tirerack does tire shaving but unfortunately they don’t sell Kelly tires. And tire shaving services are hard to come by these days. If you can’t repair this tire, you may just have to bite the bullet and buy four new ones. This issue is one of the real pitfalls of owning an AWD car.