I am also looking for this exactly for the same reasons.
I have been using TENS unit as a tuner for past 1 month. Since I haven't used original tuner, I can't compare. But I sincerely believe this to give same results. I think that frequency of the pulse doesn't have any effect on the body. There is little difference in internals and functioning of a TENS unit and the Tuner, but it looks like that tuner's price is bloated due to its lower sell volumes.