Tiffen makes an XL grad that is glass, and fits the P system, too. ~$150 (Alas, mine broke.)
Biggest problems with the P system are that the adapter rings are soft aluminum and the larger sizes are prone to bending (which makes them hard to install and can lead to cross threading), and that the frame holders only work with wide angles without vignetting if cut down to hold just one filter. The relatively expensive glass Cokin polarizer is a bit difficult to turn, as well (also tends to vignette with my smaller format 77mm threaded wide-angles).
Relatively inexpensive though if you don't buy a bunch of Singh Ray or Lee resin filters. The special effects ones are a waste of money if you're scanning and tweaking digitally. The point of a filter nowadays for me is to keep the contrast (Subject Brightness Range) within bounds of the DR of the film-- and that's it.
Now that I'm using C41 neg film for high-contrast stuff, I've blown off everything but the polarizers. Dedicated polarizers like B+W are much nicer than the Cokin one, and more neutral.
Oops... I misread. Thought the original poster was looking at using B+W filters, not filters for black & white films. Some of the above yet applies.
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