I believe the only slipper type clutch that was ever built for the RFS motor platform was from Rekluse, and it's been discontinued for quite some time. So you'd have to find a used one someone was wanting to sell.
Rekluse has moved onto the radius XC and core exp 3.0 for the supported select models. Both are auto clutches. Only the z start is available for KTM rfs engines, and between the bikes and quads they are the same besides the center hub.I was looking into this as well. They don't list them on Rekluse's web site, not even a parts diagram is available, which tells me that they don't support them anymore. This makes me want to avoid it in case there are problems and parts are needed.
I run Rekluse's in my dirt bikes and they are great...the newer ones still give you the same clutch feel when you want to use the clutch manually. I had a bike with a z-start pro and the problem was that the clutch feel is really non-existent - it's super light. That is the one thing you need to consider if you want to hunt down a used Z-Start. If you do want to use and modulate the clutch manually it feels odd/weird.