Have the very same problem for nearly two years now. A good auto electrician told me they don't do them as they are sealed units or something along those lines.
I'v gotten used to it now so I never let the drivers side window down fully, always leave a bit to grab onto if it needs encouragement to go back up. I let it down a few times without thinking and I found that opening and closing the door a couple of times seemed to jolt life back into it.
I reckon most second hand units will eventually suffer the same problem so I'm looking around for a new replacement unit. I wonder would any of the other BMW series units be made fit and do the same job?