Mitch is basically correct, a little over a year ago, Futures separated out the quad rears from most of their lead fins, and stopped selling them as sets. They sell a variety of rear quads so it makes more since for manufactures and retailers to pick and choose the rear fins they want. They actually sell them as just Vector 2 lead fins (but if you are a dealer or manufacture they usually ship them as a 3 fin set, because they produce them a thrusters in the mold). They do not need or want the extra center fins.
Cal marine can hook you up. I would strongly advise going to a premium fin. I am partial to my fin. You can use the link Drew provided.