Garmin sell their Edge computers as a bundle containing all the goodies you need, or as a standalone system. The Edge 800 costs a pretty penny, but if you’re not worried about cadence and the GPS Speed will suffice (it can measure a little lower in twisty singletrack) you can purchase just the computer safe in the knowledge that down the track you can get the Speed/Cadence kit if you want. For those who purchased a bundle, the GSC-10 is available by itself so you can have one set up on every bike you own, ready to just place the computer into its mount and you’re all set.
The GSC-10 Speed/Cadence Sensor attaches to your non-drive side chain stay and takes a measurement from a spoke mounted magnet for speed and a crank mounted magnet for cadence. Set it up so that it is close enough to the crank to read the cadence magnet and then adjust the arm to read the spoke magnet. The GSC-10 is completely wireless and uses a readily available battery.
Monitor your pedaling cadence as you ride with this self-calibrating,
wireless speed/cadence sensor. It measures and reports your pedaling
strokes per minute, providing feedback for optimal performance.
You even can use it to train indoors because the sensor attaches to your rear wheel.