The clip itself is not replaceable; it's in integral part of the plastic support end of the extender. But the support IS available from Ford:
Part #1L5Z-99286B24-AA; should run about $20.
Don't know how often you're around Frisco, but Novato Ford (north side of the Bay) shows one in stock (415-897-3978). May be one nearer to you, but that's the closest one that shows on the net.