Can someone recommend one for me. I know the switches are repeaters but I was looking for something small to plug in. I thought the aeon labs siren was a zigbee & zwave repeater but I can’t find that anywhere.
Thanks
The only single device that I know is both a ZigBee and Z-Wave Plus repeater is the Iris Plugin Module. It is an on/off device rated for 12A.
You’ll need to pair both radios. The ZigBee device controls the switch (and reports usage), the Z-Wave device is just a repeater.
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the aeon siren is z-wave, so it only repeats that. as jhamstead said, the iris plug is the only one i know that repeats both.
Is the Samsung smartthings outlet plug zigbee? I’m having an issue with 1 sensor, which is lowes and I’m thinking of adding an extender to see if that solves the peoblwm.
yes it is.
Ok thank you.
Any new recommendations for this? I have a smartthings motion sensor that goes offline (and stays off) about once a week, and considering this to try and improve it
You are replying to a post which is two years old, so there are definitely new recommendations since then. 
Since the smartthings motion sensor is zigbee, you will need a zigbee device that can repeat.
At the time of this writing, July 2019, one of the most popular is the IKEA Tradfri plug-in pocket socket. This is just a simple on/off device, no energy monitoring or special features, but it is available in many different countries and if you buy it directly from IKEA, it is only $9.99 in the US or £9.99 in the UK. 
See the FAQ for more options:
FAQ: Which devices work/double as a Zigbee Extender? (2018)
That said, while it’s always a good idea to strengthen the mesh, The “going off-line” intermittently issue is more likely to be a different, cloud-based problem. Usually if an additional repeater is needed the sensor either doesn’t seem to work most of the time or it goes through batteries very quickly. The “smartthings marked it as off-line” is a different issue having to do with smartthings’ own device health” feature and is much harder to both diagnose and fix.
So while it doesn’t hurt to add another repeater, I’m just not sure it will solve your particular problem. 