As mentioned, I suspect most people will have power from pedals, cranks or spider that gives cadence. However, Garmin do a magnetless cadence (and hub speed) that would work for you.

Garmin Cadence Sensor 2 | Bike

ETA: May want to just check the clearance between crank and chainstay. My SC is still on it's way, but noting people fit powermeter cranks with 'pods' on there I am assume the cadence sensor should fit. 9mm is what you need.