FWIW, I don't know where to get one the cheapest.
But inquiring minds want to know, what are you doing that requires a Spec Stage 3 clutch? And, if this is for a T5, which presumably it is, I can assure you that you will be posting another thread in a short time after the clutch install asking which company sells the cheapest T5 rebuild kit.
$0.02.
Joe