Where can I buy a new cobra immobiliser thingy? (& what are they called??

I don't know what they are called but the cobra barrel thing that you put into the round hole on the dash when your security system needs resetting, has broken off of my key ring and been lost. I have a spare but obviously want to replace the missing one.
Does anybody know either, where I can buy a new one or (as my dad has the same car and has an old spare one) where I can get one reset? Also what are they actually called :) ?
Thanks