I bought a TENS 7000, which is #1 in Amazon's "Muscle Stimulators & Accessories" category. Mine came with a 9V battery. The specific "tens ear clips" that I purchased are no longer on Amazon, but similar ones are around $20 for a pack of 3-4.

Note: some people place electrodes on the tragus, as in this paper, but others stimulate the concha. Tragus needs one two pad clip, concha needs two one pad clips. The rubber pads are just slightly bigger than the eraser of a number 2 pencil.