I bough mine locally from a fish store (not a chain). A chain store is a large chain that operates nationally. There are also some smaller chains that are regional. Then there are stores that are independently owned that generally have one or a handful of locations; this is what I think of when I hear the term "local fish store."
I think they generally have them available at Dr. Foster & Smith and That Fish Place if you want to order online. I'm sure there are other online options. You might check Aquabid also.