Ansel Adams recommended the 47B, and after looking at the manual on filters, I can see why. Unfortunately, I don't know the B370, and it doesn't seem to be in the manuals I have.

If you go through B&H, I know they have them available, and Amazon has several from Tiffen listed in different sizes, in prices ranging from about $20-$100. In fact, I may have to get 1 for me as well, for when I'm shooting in the shade without sky in the scene!