I wouldn't use them for everyday riding without checking the sole first - the sole might not be suitable for stirrups. But in a controlled setting like a show, they should work just fine. If you go with a well known brand, like Harley Davidson or... well, that's the only brand I know, LOL -- anyway, a well known brand should hold up fine.
I was thinking of those harness motorcycle boots. A lot of Western boots look real similar to them:
Edited to add: The above boots are the "El Paso" model of Harley Davidson, and they come in a size 18! They also have black boots.
Might be worth it to look at Blundstones. I just checked their website but they didn't have any sizing information that I could find. Although it's getting late and my mind is foggy, so who knows. You could try contacting the company. And they aren't too expensive, running at under $200.