I would ask the store where I purchased the other ones to order it for me. If they have multiple locations, they could check there first, and if not, they could order it from their distributor (and shouldn't charge you extra for doing so).
I'm surprised they didn't offer to do that in the first place. Of course I am the type of customer that would say to them, "Well if you can't get me as many as I need, then I'll buy ALL of them somewhere else"