Garmin has a new cadence sensor that instead of 2 magnets, this new one only uses one part that you can attach to the crank of the bike. However, you will need also to buy the Garmin watch to pair with it:
Check this video to get the model and how it works. https://www.youtube.com/watch?v=h8wswjbZXzo
That may help you set up quickly in the gym, at the extra cost of buying a Garmin watch.
But, while Googling whether you could use this Garmin sensor in your shoes, I found this review and I think this may answer better your question: http://www.dcrainmaker.com/2014/01/r...formation.html
It seems there are already bluetooth device that you can just wear in your shoes and no need to buy a watch, (but you need a cellphone with bluetooth).
Hope it helps,