What are you trying to achieve?
Lifting or sensing 
Dangerous things xG3 is designed for lifting ( I don’t have one so can’t comment personally on performance but you can search this forum for other peoples results example )
Dangerous things no longer sell their M31 sensing magnets, and I don’t believe they ever made the M36; nor do I know if they plan to do anymore in the future, watch this video
But it might be worth having a look through these results to at least get some more information from the forum around implantable magnets in the community.