There are several UK vendors listed in the following thread:
The smart things – branded motion sensor should itself still be available from the Samsung site listed there, for example.
More importantly, though, the SmartThings-branded motion sensor is a zigbee device. In the UK, you will have a lot more choices of both devices and retailers if you go for a Z wave motion sensor like the fibaro or Aeon labs. You should be able to find both of those at Vesternet or Amazon.co.uk
If you do want to stick with a zigbee motion sensor, the Orvibo zigbee model is working quite well with community-created code. However, the only place I know that is selling it is Banggood , and not everyone is comfortable buying from them.
You will find much discussion of the Orvibo contact sensors in the following thread, with some discussion there of the motion sensor as well.