Only just sold two of these so cant help, but there is plenty on ebay, you could also use a SAAB 900 remote fob with your x300 and sometimes they are easier to find and are cheaper. I use one for my own x300.
1997, 4.0 LWB X300 - Unlucky car but still a daily driver.
1988, 3.6S XJS - Never ending restoration project.
1994, 3.2S X300 - Accident damaged, October 2015. Gone!
1994, 3.2S XJ40 - Long gone.