Not expensive, but I suppose it depends on where you get it from.
I believe for a TENS unit you need a drs signature /Rx or possibly a chiropractor would work as a signature.

[PURPOSE: A Transcutaneous Electrical Nerve Stimulator, or TENS, unit is designed to relive post-operative, acute and chronic pain. It is used for pain caused by peripheral nerves and not central. TENS units are prescription-only devices. ]
http://vitalityweb.com/backstore/tens2.htm

I got a EMS machine for my chronic mild muscle pain.
You don't need rx for that kind.

more info on the various types of e stims-
http://vitalityweb.com/backstore/Muscle-Stimulator.htm

tens unit reviews -
http://www.google.com/search?hl=en&c...S+unit&aqi=g10