I got mine at Dick Blick as well. I seem to remember they were the only ones I could find selling sheets instead of rolls. A couple large sheets are probably a lifetime safelight filtering supply for me.
That's why I offered to just send the OP a small piece. Brownie red dot windows are awfully small to be cutting from a 20"x24" sheet...
"When making a portrait, my approach is quite the same as when I am portraying a rock. I do not wish to impose my personality upon the sitter, but, keeping myself open to receive reactions from his own special ego, record this with nothing added: except of course when I am working professionally, when money enters in,—then for a price, I become a liar..."
— Edward Weston, Daybooks, Vol. II, February 2, 1932
Perhaps if we knew for which brownie the window was needed or the approximate measurement for the window, there might be some other suggestions. I have a brownie or two that might be able to be donors with a little more information.
Get a slide film, shoot a couple of frames aiming at a uniform red surface (chose the apropriate tonality of red), and enjoy the rest of the roll aiming at your favourite subjects. The red slides can be cut as you want to replace the red-window.