Any Mains-powered zigbee device that is controllable by smartthings should be able to act as a zigbee repeater for your network. Typically this would be a plug-in pocket socket like the SmartThings-branded Smart plug.
Some UK community members are using the less expensive orvibo pocket socket. You can read about their experiences in the following thread:
Note that orvibo makes devices on a number of different protocols, it’s not enough if the device is just as zigbee, it has to be ZHA 1.2 certified in order to work with SmartThings. So make sure you’re getting the right model.
I think there’s another brand available in the UK, I’ll see if I can find that one.