Here:
"The Ultimate Easy Guide To Neutral Density Filters" by Peter Hill | RedBubble
B+W seem to have ND #110 in many sizes reasonably priced also. Apperently it does give a magenta cast as do many other filters.
Hoya have ND 400 (9 stop). Cokin, Hitech, Tiffen, Kodak + Lee also do a 10stop
Lee Big stopper & Hitech review here:
Lee Big Stopper and Hitech 10 Stop ND filter Review
"twitch" has a B+W which he featured recently. DA15 thread page 149
"digitalis" has a Lee Big stopper among other things.
https://www.pentaxforums.com/forums/lens-clubs/86234-15mm-limited-controls-my-mind-club-149.html
You can see there is a colour cast on the B+W so shoot in RAW is the go & pp later.
edit: about 1/2way down reading the ND finlter guide link above, he mentions the colour cast for these filters.
Quoted from "
The Ultimate Easy Guide To Neutral Density Filters by
Peter Hill"
"
Can you guess which is which? The Brown job is from the B+W, whereas the Blue job is from the Lee. Colour casting is inevitable whichever of the 2 you use. Using a Cokin or Tiffen or HiTech ND filter will also give you colour casting, but at much much lower densities. For example, Cokins give a pinky caste, whereas HiTech is much darker. The only exception to this that I have experienced is that the Hoya ND x400 Filter – The Black Glass – does not
normally produce casting (actual copies are variable), but sometimes flaring. (If your Hoya flares, check your screw-in and if it’s screwed in tight you’ve got a bad copy.)
The colour casting is fixable, but
only if you’ve
shot in RAW. Of course, if your final image is in black & white, it don’t matter diddly. It is however rather niggly to have to tweak your White Balance in the conversion of a RAW file, especially when you have a Canon 5D Mark II, like I do, a or similar camera which does an excellent job of handling Auto WB itself under normal circumstances, as I mentioned above."
Hope that might be helpful