WeMo mini is quite good if you’re ok with WiFi, and it seems to go on sale quite often. It’s designed specifically for stacking. Should work out of the box with SmartThings, although you have to install it through the Wemo app first, then wait 10 or 15 minutes, and then as long as you’re on the same Wi-Fi network as your smartthings hub you should be able to add it to SmartThings. It’s become quite popular because of the sales.
https://www.amazon.com/3-Pack-Enabled-Amazon-Google-Assistant/dp/B076J8SLSF/
Amazon has a new WiFi plug that you could set up via an Amazon routine:
https://www.amazon.com/Amazon-Smart-Plug-works-Alexa/dp/B01MZEEFNX/
Breaking News!!! As of August 2018, a SmartThings-connected Contact or Motion Sensor can now trigger an Amazon echo routine if you live in the US, Canada, or the UK !!!
Amazon has now added the ability for some sensors to trigger an Amazon echo routine (not a smartthings routine), which does let you trigger other smart home devices which are not connected to SmartThings without having to speak an echo command! Or you can set it up so that your echo devices will speak a customized phrase, play a doorbell chime, or various other sound effects.
For a different approach, Zooz has a zwave plug with two independently controlled outlets that might work for you.
https://www.thesmartesthouse.com/collections/zooz/products/zooz-z-wave-plus-s2-double-plug-zen25-with-usb-port
And GE has a similar design.
https://www.amazon.com/GE-Individually-Controlled-SmartThings-28177/dp/B0736311QF
