T here were 2500ish automatics registered.
If you want a 50k mile car you are looking at cars that have done on average 5k or less per year. There will be a miniscule amount of cars for which this is case, and even fewer will be for sale. It's going to be like looking for a needle in the world's largest haystack. How many people do you think spent £35,000 on a big executive saloon and then barely used them? A few people but not many. Most of the old guys who bought this sort of car and never used it bought an SE not a Sport.
What's more I'm not even sure what the point of it is. An ultra low mileage E39 isn't going to protect you against bills and neither is it going to be a world away condition wise from cars with higher mileage. I sourced a low mileage 530i Sport Touring (Under 60k) for my parents but only by co-incidence, I didn't care what the mileage was and it happened to appear. Compared to my lolmileage example there is little to tell them apart on the inside - my driver's seat is a bit more worn, my passenger/rear seats are infact far less worn and generally my interior is a nicer despite having 3 times the mileage (My rear seats still look virtually as new!). Externally mine is better as well. Infact the wear is consistent with what you'd expect on a car that's spent almost 10 years popping into town etc with its single owner from new. Tidy but not immaculate.
IMHO the key to a nice E39 is finding a rare 1 owner car and not necessarily a low miler. Whatever you buy you must accept these are 9-12 year old BMW's and WILL require bits and bobs replacing to freshen them up and will drop bills in the future. The way to avoid a shed is to ignore the 3-5 owner cars with cheap tyres, cheap servicing or dodgy mods and not fixate on the mileage.
If you are obsessive about low mileage buy something much newer as that's the only easy way to get low mileage.