Right now the cheapest plug in either the US or UK is the IKEA Tradfri zigbee plug. $9.99 in US and £9.99 pounds in the UK. Works well with smartthings and is a good zigbee repeater, but it’s just a single on/off pocket socket.
Product description will say you need their gateway, but you can use the smartthings hub instead. Some of the initial reviews were really low because people were expecting it to work with HomeKit like the rest of the tradfri line, and it doesn’t yet. But looked at just as a zigbee device, it’s fine.
Alternative: Echo as a “man in the middle”
Alternatively, any device that works in an Alexa routine (not a smartthings routine) can now be controlled from smartthings. Setup is a little bit complicated, but after that it works fine. 
So if you can find a Wi-Fi power strip that shows up in an Amazon routine, you can use that. Or individual pocket sockets.
See the FAQ (this is a clickable link)
IFTTT
Or, yeah, you could just Google the Wi-Fi strips that work with IFTTT. Like this:
https://www.amazon.com/Multi-Plug-Protector-Compatible-Control-Android/dp/B07KW2T5YT/