Bmw dealer is the place unless you can locate one from a wrecker's yard.
Found this on another forum:
Before you will proceed with replacing the actuator, check the trunk button because
it very often becomes jammed in closed position, screwing up the lock logic. It
happened on mine. You can either remove (using small flatblade screwdriver) whole plastic
assembly with rubber button or alternatively pry out the rubber but then you will have
to watch for springs and plastic parts falling from inside, which you have to collect and
reassemble.
If the button is OK, then this is the common actuator problem they are easy to do all will be come obvious when you see your new actuator