Here are some sources for known good magnets suppliers. Some controversy may exists over who is reputable and which is the "best" magnet. This portion of the wiki is not for those discussions. These are places that have been known to supply magnets that are implantable and have not caused issues.
Fellow grinder Amal Graafstra and SfM collaborated to create this magnet and designed it specifically for sensing.
Steve Haworth Modified, LLC
Steve Haworth was an early pioneer in "magnetic vision" and designed the first magnet for implantation
V & P Scientific
These are magnets found early on by the community. They are known to work and are safe to implant, however they are not designed for this purpose.
Alex smith runs this site and can often be found on the forums. These magnets are safe for implanting. (Not offered, currently in testing)
Cheap gold plated M31 magnets sold on Ebay similar to the ones sold on dangerous things. Ships to the UK, America and Canada.