Dave,
You can also cut your own from one of the new molded Para-rest brush rests, available from 3rivers, Joe St. C, and others (about $3.50 ea). The brush portion isn't quite a long as that on the original Bear rest, so it won't quite fill the entire length of the slot in the sight plate, but they work very well - especially if you can't find an original. Just take your time cutting the molded plastic of the Para-rest in a wedge shape (along the top and bottom of the brush itself), and then feed it through from the back side of the sight window plate and press it in place. I have replaced all of my original Bear brush rests with these modified Pararest replacements so that I don't wear out the originals.
Ron