There are actually two different versions of the fender skirts for '41: one type for 50 and 70 Series, the other for the 40-60-90 Series. While they will technically fit universally, putting a set of 50-70 Series skirts on your Special will not result in a perfectly flush fit, particularly at the rear of the skirt where it may protrude. The rear fenders on '41 Buicks are all about the same size, with the only differences being the subtle shape differences between the 50-70 and 40-60-90 bodies. The B-series Specials used a lot of unique pieces, so I don't know if the rear fenders are interchangeable with the other cars or not. I would guess not.
The aftermarket skirts that you're looking at are usually generic catch-alls, so they don't fit any car particularly well. Also, the shape of the '41's rear fenders is completely unique--one year only (although '41 Cadillac fenders look very similar and might be able to be modified to work with a Buick). I've seen aftermarket units in fiberglass that were the right shape, but never in steel. If you want steel, it's probably OEM or nothing. A universal fit set for '41 to '46 GM vehicles (I would think '42 through '48 would be more interchangeable than '41 through '46) won't look right at all. The '41 Buicks had a squared-off fender shape that is echoed by the shape of the skirts--they aren't really rounded, but more like an oval that has been flattened on the back. There are plenty of '41s out there with the incorrect skirts (most frequently from the '39-40 cars), and they don't look right.
This car has skirts from a 1940 Buick installed with a '41 ornament and no stainless trim. You can see how the upper edge of the skirt doesn't follow the curve of the fender's crown and the shape is all wrong on the lower edge. The stainless trim is not present, either. However, if you're not doing concours judging, they do look decent, just not correct.

Remember that installing the skirts will also necessitate the use of special stainless trim pieces along the lower edge of the skirt and at the bottom of the rear fender stone guard (again, this little piece is unique between the 50-70 and 46-60-90 cars). These little pieces are EXTREMELY hard to find, and you will see many cars with skirts that do not have this trim installed. It is designed to continue the rocker panel stainless trim all the way to the back of the fender skirt. This interruption at the stone guard just looks odd.
You can see the correct trim pieces installed on this all-original Special 46-S (the tiny trim piece is visible at the base of the stone guard), although this car seems to be wearing 21" side spears. Also note how the shape of the fender and the skirt are roughly the same.

There is also some considerable debate over the size of the side spears, which were available in both 21" and 24" sizes. Generally held consensus is that the 24" spears are for the skirts, although there are a lot of cars with the 21s on their skirts that look just fine. I have some 24s that I was going to use on my un-skirted car, but it appears that the very rear points of the spears protrude from the fender. I'd say 21s are correct either way, 24s only work on skirts. Note that the fender skirts will not cover all the holes in the fender where the side spear was originally mounted. You'll have to fill these and repaint the panel in order to use the skirts.
If you want real 1941 Buick skirts, be prepared to pay dearly for them. I've seen good sets go for $1000 on eBay, and even a single rough specimen without any trim has sold for $400.
Hope this helps and good luck with the search. That's half the fun!