In Japan primarily, as they're highly sought after. They're not at all readily available at all, as it's usually a personal connection that gets you them.

Not to mention very expensive even without the shipping costs and potential phytosanitation certs. There is a grower in the Netherlands, I believe, that has an amazing assortment of Haws.

Here is a link that might be useful: Succulent Tissue Culture

Too bad. I had no idea they were THAT exotic! Guess I'll just have to look at pictures of them.