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Its like a string so you don't need to remove the cassette; http://www.amazon.com/FiberFix-Emergency-Spoke-Replacement-Kit/dp/B001GSMQZC
Spokes failed midpoint which is unheard of apparently - I did have a slight issue with the derailleur just catching the spokes in lowest gear a few weeks back, but quickly fixed and there was no apparent damage.
Thinking about it I have had an occasional "pinging" noise ( that I thought was from the front but could have been the back) on really rough surfaces, so the spoke tensions may have been out?
Kevlar repair spokes? Cool. I'd pose the question why were you breaking spokes in the first place though? Are you running silly light wheels? Was the build less than perfect? Were they crashed? Spokes tend to just fail unless there's an issue already.