Short answer: Use the one Kinesis say to, and when the bearings wear out (use lots of grease when you assemble everything and they will last longer) replace bearings from a decent local bearing supplier.
Long answer: Despite this being quite a normal headset standard in road bike circles, basically there’s way too many options out there, and the risk of buying the wrong thing (bearings may only be 0.2 or even just 0.1mm too big but won’t work!) is too great to bother IMO. You’re not really buying a “headset” as we traditionally know them anyway, merely 2 AC Bearing, a crown race and a top race. Any decent bearing supplier or bike shop should be able to furnish you with half decent quality replacement bearings as they wear out.
FWIW, I’m running a Sync, the GF has an FF29, and my Dad has a GF Ti all with the same headset in, and no issues to report as yet.