Etsy shop, rather than in sets of six.
While this means that I can't guarantee that you'll be able to buy a matching set of six buttons, it also means that you're able to create your own combination packs. Or even buy just one button.
Although the shipping is costly for just one single button, you'll pay no more to ship twenty than you would ten or six or one. And no shipping at all on that one button if you buy another item!