What country are you in? The device selection does vary. In particular, there are a few of these made for the EU market, but they might be hard to find in the US.
BENext makes a zwave device which is a 4 digit keypad that also reads RFID chips. It’s available on both the US and EU frequencies. It has a custom DTH that worked with the Classic app; I don’t know if anyone’s updated it for the new V3 app. That would probably be the easiest, but the trick is going to be finding one for sale. Also sold under the Zipato name.
Zipato / Benext / Wintop RFID Mini Keypad
If you want to go the DIY route, you don’t need an arduino or raspberry pi in the mix. Just wire a zwave sensor to detect the output from the
RFID reader. See:
And this thread should be helpful
FAQ: List of devices with dry contact input from external sources
There are a bunch of Zigbee RFID readers made for security systems (including Control4), but most of them will NOT work with SmartThings as they use a different zigbee profile.
Develco makes a zigbee 3.0 which might work, and you can purchase a single unit from their sample shop, but as far as I can tell it’s only available in the EU, it’s quite expensive, and it would need custom code. So I’m mostly just putting it here As POC, but I think the DIY method would be better for most people.
@rboy is an expert on locks and keypads and might know of something else.