I have only seen them on eBay. They are pack-ins to some kind of candy or snack in Japan only.
What they get in each box is a mystery I do believe. So they have to hunt them down, and you get a little box for each craft that has already been opened, and the model is a snap-together in a little bag.
You can usually get them from a west coast eBay e-tailer, or a reputable seller (one with 100% positive feedback recommended) from Japan. That's how I got all three waves.