I don't know about cadence sensors particularly - will any magnet do?

Radio Shack has ceramic and rare earth magnets, $2 for two. They're super strong - I've used the rare earth magnets for tendinitis and they work really well.

Alternately, I have two spare Garmin cadence sensor magnets - they've had to replace the sensor under warranty twice, and the package comes complete with magnet, which I don't need. PM me and I'll mail you one.

Whichever is more convenient...