Front Passenger Seat Armrest

So one of my wife's gripes about the 5 is that her seat has no armrest (I have an 07 Sport). I've seen a couple pics of an 08 that had an armrest on the front passenger seat. Anyone know if there is an add on piece that will just fit the seat that is there or do I have to replace the entire seat? If I have to replace the seat and 08 up has the armrest, to they just fit right in or are there changes to be made. Just looking to see if anyone has done this so I'm not recreating the wheel. Thanks.