TENS Units (Transcutaneous Electrical Nerve Stimulator)

I need to purchase a TENS / EMS unit. The specifications I’m looking for are the following:

Low frequency
Pulse width maximum 400
Pulse rate maximum 150
Two channel (four channel would be okay)

I would prefer it operate on either standard AA, AAA or 9 volt batteries. I definitely do not want any units that need to use proprietary electrodes or lead wires or can only be recharged by a USB connection or a plug-in power supply.

I have found several models that meet the specs. If you have used a TENS unit with these specs please let me know what make and model . Also let me know if you were happy with it.

Thank you for your help.