Pretty much any zigbee device that is not battery powered except some ZLL lightbulbs will work as a repeater. To be compatible with SmartThings it should use the HA1.2 zigbee profile (HA= Home Automation). Not Home Automation Pro, which is different.
The Osram Lightify lightbulb, SmartThings original motion sensor (but only when it is plugged in to USB power), and the plug in zigbee SmartPower outlet all work. So will the SmartenIT wired relays, but not their battery operated switch. The light bulb is the cheapest, but if someone turns it off at the switch it won’t repeat.
Hue and GE link lightbulbs will only repeat for other bulbs.
After you put the repeating device where you want it, unplug the hub for 15 minutes, then plug back in, to make sure all the repeating device’s neighbors find it.