Make your own out of a scrap Polaroid print. Use scissors to cut out the image, leaving the white border. Tape the white border over the back of the groundglass using 2" of that good, blue masking tape that never leaves a residue at either end of the white cutout. Easier to do than describe.
You might want to pull off the back of the camera, insert your Polaroid holder, and see whether the image area is centered or favors one end or the other. My Wista cheats a little towards where the holder is inserted, so my paper cutout isn't centered over the groundglass.
When the paper from the scrap print tears, wears, or falls off, make another.