I know for a fact that I have purchased 18 and 24 there I cannot speak to the 13" . They are usually placed near the other Fluro bulbs but are usually right beside the full light packages (bulb and ballast). They come wrapped in a small square cardboard box.
That looks a lot like the type of bulb found in an aquarium hood. This is a long shot, but perhaps try an aquarium shop. Be careful of the colour temperature, however, as there are a number of varieties for things like coral, vegetation, and fish.