Welcome to the forum!
Here is an index of the most common locations: Implant Sutra - An index of implant positions that have been discussed and/or tried
You can also check out my book if you are new to magnets:
In general avoid having it over a bone, too close to a joint or in an area that receives impacts or gets pinched when holding objects.
I always recommend to tape/glue a small object the size of the implant on the planned location over a day or two and see how that work in your daily life.
If you are looking for a deep dive into biomagnets and the science behind them here is a collection of my previous research and projects.
It would be helpful if you describe what you intend to use it for (sensing or lifting or something else) and which one you plan on getting so we can better help you.