Medical supply stores and sex shops sell them. You can get them online. However I would advise extreme caution in using a Tens unit above the neck. While it is not a strong current is is a direct and deep one. It could put you in a seizure you cannot stop or give you a stroke if used incorrectly. If you feel you have no choice but to self-electrocute it always has to be under the direct supervision of an adult, preferably one who knows CPR.

The TENS unit is one that is currently being used daily in a neuromuscular dental practice. Simply put, a dentist that has been trained in treating TMJ disorders through electro stimuli of the facial muscles. I'm a dentist and I use it in my practice. Its rare such a unit would be accessible to the general public, because the company's liablity woulb be beyond belief. There is a skill and science to proper treatment using the TENS unit, and cheek muscle pain has been known to resolve after treatment of this device.

I got one quite easily through my physiotherapist.