You can use any button controller that works with SmartThings – – just have each button run a routine and have the routine change the SHM armed state.
The Aeotec 4 Button key fob is available on both the US and the UK frequencies and is officially supported, so I would probably just go with that one, but there are others you can look at as well.
The original model has been discontinued, but the newer Z wave plus model is widely available and at a lower price.
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https://support.smartthings.com/hc/en-us/articles/202294120-Aeotec-Key-Fob
It’s a little tricky to get paired the first time, but once you do it should work fine.
If you want to look at some other options, see the Buttons FAQ,