I know your pain.................... BMW do have them in stock but they don't know a part number so you can't order them. My local main dealer parts counter are excellent and go to great lengths to help but they have been frustrated many times with older accessory parts.
It took me ages but I finally found a set 18 months ago. Eurocar parts stock them, they do not know the difference between the sill finisher or the standard sill so when you talk to them they are not much help. They just state it is a part number for late E30 BMW front Touring mudflap. I ordered a pair and just waited to see what turned up. What arrived had the cut out for the plastic sill finisher and as an original accessory part they fit perfectly. They cost about £30 for the pair.
If you get a pair do not drill the hole in the sill when you fit them. They still fit nice and snug just using they other fittings. However, if they ever get caught up or ripped off, as mudflaps do from time to time, they would damage the plastic sill finisher if they were pulled off.