Are you in Europe or the US? The device selection does vary somewhat.
In either case, almost all mains powered zigbee devices Will act as zigbee repeaters. That includes plug-in pocket sockets, plug-in sensors, in wall relays, etc.
You just have to make sure that it is using the same profile as SmartThings, ZHA 1.2 ( zigbee home automation)
Once you tell us what region you are in, we can suggest some specific devices.