Hey bini, welcome to the community
You may be talking about Rich Lee. Several other biohackers around here are still experimenting with that as well. Pretty much any small sensing magnet should work. Unfortunately most of the smaller magnets that are available are not very trustworthy, and the coating fails after a few years. The only magnet that is available from Dangerous Things currently is the xG3, which is a bit large for this job.
Here is a thread about tragus implants and using them as headphones
Currently I don’t have any recommendations on where you could get a sensing magnet.