“Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”
Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2
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...
"They are the proof that something was there and no longer is. Like a stain. And the stillness of them is boggling. You can turn away but when you come back they’ll still be there looking at you."
— Diane Arbus, March 15, 1971, in response to a request for a brief statement about photographs
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.