Sheila's on the right track. I think that they are all moving toward the combination units, with both TENS and EMS or other stimulation. They will probably be phasing out the older TENS only units.
I also got a lot of hits on searching that insurance companies are no longer covering them, so that may be an issue too. The home machines are probably cheaper than going through insurance anyway if you have any kind of copay.
I hope you find what you're looking for!