It’s mentioned in the no neutral FAQ, but just in the sense of mentioning that that’s an option.
There are several listed on the official “works with SmartThings” list.
Both the Evolve LFM and the Aeotec micros are popular. The evolve is usually a bit cheaper but has fewer features, but should be fine for just on/off.
https://www.amazon.com/Evolve-LFM-20-Controls-Z-Wave-Certification/dp/B006U1O978
https://support.smartthings.com/hc/en-gb/articles/202294140-Aeon-Labs-Micro-Smart-Energy-Switch
Monoprice has an even cheaper dual relay for about $30 which can control two separate switches on the same circuit. Some community members are using that one, although it will require custom code and is not on the official compatibility list.
http://www.monoprice.com/product?c_id=122&cp_id=12212&cs_id=1221201&p_id=11990&seq=1&format=2
So there are several brand choices, but they all basically work the same. There has to be a neutral on the circuit somewhere, and if it’s not at the switchbox it’s often at the light fixture. So you find a place where the neutral is and you install the micro inside the wall there. (Or maybe inside the fixture) you can still use the switch at the wall, although most people would replace that with a momentary button switch, more like a doorbell button (although they come in many different styles and sizes).
Option 2 in the UK lighting FAQ discusses relays in more depth since they are often the only option for a house with European wiring. It’s the same devices as the ones sold for the US, just on a different Z wave frequency.