Amazon with Smartindex has the RENPHO Wireless TENS Unit Muscle Stimulator w/ 4 Electrode Pads for $21 - $1 off with coupon code PLEW7I8W at checkout = $20 with free shipping on $35+ or with Prime.

The 3-in-1 Tens Unit is also on sale for $20 after code YQXRRN7J.