I've used all sorts of things for that. An old pair of headphones provided numerous fixes for my cameras. Just trim off the proper piece from the foam padding and it works fine. I used that pair of pads up finally, and on my FE-2 I cut a piece from some thin black strips of weather sealing I had for going around doors. Worked just great. This is one of the easiest DIY repairs you cam make on a camera. The first time I did it I was disappointed that it made no difference in the mirror slap though. That seemed to come when the mirror slammed back down. Nothing to be done about that.
....and in the narrow channels for the back and other places I use black wool.
For the mirror and prism seals, and most uses, I use the foam from Jon Goodman and it seems to be an exact match. But for the back light seals, I'm using black yarn, as E. von Hoegh suggests. I have an original Nikon F from the late 60's that came from the factory with black yarn for back seals, and it is still good as new.