Contact them. Ask if they ever put 6 or 9 of the 4 dram vials up for auction. I wouldn't necessarily buy directly from them (unless they have a website where they also sell, there's no protection or accountability), but there's nothing wrong with encouraging them to create an auction tailored to your needs.
Of course, if they charge $15 for 12, they're going to charge $9 for 6. If you are going to need the other 6 eventually, you might as well buy them now.
