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Unfortunately Stop Tech does not sell the kit for the Slingshot other than through their vendors. You can get replacement rotors for the kit but that does you no good without the custom hat which is where the cost lies.
I will agree they are overpriced since they are the "Sportsman" standard pillar construction and edge balanced not the higher performance "Aero" rotors as alluded to by a certain vendors advertising and word of mouth. They are also only available in gold zinc coating not a choice of silver or gold as advertised. If you go with them they are directional so make sure you receive two lefts and a single right ..... the vendor shipped me three right side slotted rotors and I had to explain the mistake to them so it was pretty evident they had not been checking and the person shipping had no idea.
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I know they don't do any direct sales, I usually get all my centric stuff from autobrakecenter.com they have cheap prices, if I can get the part number I could get the rotors cheap there.