[UPDATE] Turns out you can get dental implant hardware on eBay:
https://en.wikipedia.org/wiki/The_Thing_(listening_device)
in the last 77 years, I imagine this type and similar technology will be much more miniaturised and effective.
No internal power supply. Very important distinction.
"The device, a passive cavity resonator, became active only when a radio signal of the correct frequency was sent to the device from an external transmitter."
But the Thing was a transmitter. A receiver could be entirely passive and very small. It could easily fit in a dental implant. (There have been reports of people hearing AM radio broadcasts through their fillings.) But the risk of detection would be very high.
