Need magnet for RPM and speed sensors
To a large extent a magnet is a magnet (so long as it's strong enough).  You may have to mount it in the correct orientation.  A spoke-mounted magnet is quite easy to make or cheap to buy. The one that mounts on the crank for the cadence sensor may be a little harder, but I've got one that just relies on fixing to the pedal axle with its own magnetism (from a sensor that's too cheap to actually work reliably). You might not need a bike shop in that case.