They occassionally pop up on ebay and sell for about a tenner. I think they're about £35 new.
It's not straight forward to program them either....
Bike ignition off and remove the inline fuse. Take both green wires, bare the ends, twist them together and attach them to a 12V supply. Turn bike ignition on, replace inline fuse and press and hold the circle button, you will hear one single beep. Release the button, count to 5 and press and hold the same button until you hear two beeps. Undo the green wires and mask (or remove) the bare ends and return everything to its original state.