Some knowlegable soul (I believe M.B.Doc) reported they are available aftermarket from Bosch - not MB. Do a search on it.
You could also have a look at its repairability I believe there are two wipers inside connected together. One would move on a metal track, the other on the resistance element. It seems unlikely the wipers themselves are worn out, or problems would occur at all speed ranges. If there is some way to shift them so they 'wipe' a new section of the two tracks, operation might be restored. This is speculation, but something I would look at before replacing the sensor. I also wouldn't risk it unless I KNEW I could replace the sensor with a new one quickly, if need be.