motion sensor compatibility
You have three different options depending on exactly what you need
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if you are OK with a set up that only works when the Internet is working, you can get any motion sensor at all that works with smartthings and it will send a message to smartthings when it detects motion and then smartthings will tell the switch to turn on.
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if you want to set up that will work even if the Internet is out as long as your smartthings hub is working, you just need a Z wave or zigbee motion sensor that works with the basic features of the hub. In the future, this will be almost all zwave and zigbee devices that work with smart things because of the new platform, called edge. So you have a lot of choices, but I would probably recommend Zooz. Zooz is the house zwave brand for the retailer the smartest house. they are moderately priced, well engineered, and very popular in the community.
They have a very basic model for just under $20, or check their site for some more expensive models that also have temperature and humidity sensors.
My personal choice would be the one with the latest and greatest Z wave technology, one that uses the 700 series. You probably don’t need that, but it does have some advantages in range and battery life. That variant lists at about $37.
- if you want a setup that will work even if your smartthings hub dies, you can use zwave direct association. This method allows direct association between two zwave devices, so that messages don’t have to go through the hub. Both the Zooz models I’m already linked to do have this ability, as do many, but not all, zwave motion Sensors. But note that this only works if the switch is also a Z wave switch. You can’t mix and match protocols with this method the way you can with the other two
The only thing about this feature is that smartthings doesn’t make it easy to set up. You can do it, but it’s a little tricky. so a lot of people settle for option one or two.
A note about zigbee vs zwave
Remember how in option one above I said you could use any motion sensor that works with smartthings? Some people, including me, have historically had a preference for zigbee battery operated sensors over Z wave battery operated sensors, because they tended to have better power management, so they could be a little smaller and they woukd react a little quicker. The question is whether that really matters in a practical sense. Zwave has been getting better and better (The new 700 series are much better), so since it’s now late 2021, I wouldn’t argue against a zwave motion sensor the way I used to. Now I think it’s just flip a coin. If you like the look or the features or the ability to set up direct association of a specific Z wave model, then get that one. 
Do understand the issue of positioning, as it will apply to either a Z wave or a zigbee sensor.