Where to buy leak detectors for ST?

Do you mean the new Aeotec version of the former smartthings Brand zigbee Device?

Amazon has it:

https://www.amazon.com/Aeotec-SmartThings-Battery-Powered-Compatible/dp/B095TR9NYR/

Samsung is no longer making smartthings branded hardware. Instead they are partnering with Aeotec in the US, the EU/UK, and Australia, to sell what are essentially rebadged versions of the same devices. So you can no longer buy the “smartthings“ brand, but you can still buy the devices.