Hasselblad Wide Angle

Your best bet is to use a gel filter in a Pro Shade, or a square filter in a Cokin or other holder. B+W does not make a 93mm filter, which is the nominal size required by the CF40/4 lens. If they did, it would cost at least $400.

In a practial sense, a polarizer does not work well on a wide angle lens. The sky is polarized in a relatively narrow band 90 degrees from the sun, which produces a diagonal streak in wide-angle pictures. A similar phenomena occurs in reflections from objects on the ground. While the 40mm is not very wide by 35mm standards, it is still wide enough (24mm equiv) to have undesirable effects.

B+W does not make a 93mm filter, which is the nominal size required by the CF40/4 lens.

I bet you any amount that they do!
(A friendly warning - i know i shouldn't, but ... - before you take that bet: i have one of those right here ;-) )

If they did, it would cost at least $400.

That however is true.

I got the Hasselblad version off eBay...and the compendium hood that fits it...you just have to watch.

Actually, the 40mm lens for the Hasselblad is almost exactly the same angle of view as a 26mm on 35mm format.

I had a SWC, and as I was told and experienced, the 38 is close to 21 in 135 format. So, I'd say 24 is a closer equiv.

They're both wide enough IMHO

Horizontal angle of view for 40mm lens = 69 degrees. (6 x 6)
Horizontal angle of view for 26mm lens = 69 degrees. (35mm)
Horizontal angle of view for 38mm lens = 72 degrees. (6 x 6)
Horizontal angle of view for 24mm lens = 73 degrees. (35mm)

I'm curious where you go to buy a 26mm lens for a Nikon? The point is that comparisons should be made to lenses commonly available, e.g., 20, 24, 28, 35, etc. Even these values are nominal, and the true focal length may vary by a millimeter or two. Then again, which angle is significant, horizontal, vertical, diagonal, cropped to 8x10? I think a rough comparison based on "normal" focal lengths is sufficient - 24mm is about half of 50mm and 40mm is half of 80mm. (If it helps, Zeiss makes a 25mm ZF to fit Nikons.) I'll leave the tenth millimeter calculations to those practicing their 8th grade geometry.

There may be 93mm B+W polarizers in existence, but sizes in the Schneider catalog jump from 86mm to 95mm to 105mm. This corresponds to the listings at BHPhotovideo. I believe the OP's question was about where to purchase one. Without Schneider or B&H in the running, that leaves flea markets and auction sites.

Any of these focal lengths are available if you have a 35mm zoom lens, irrespective of manufacturer. Regarding which angle of view, I would have thought that the horizontal figure is the most practical, as that is how most images are seen in the viewfinder. One rarely composes an image diagonally.