I know this is a very old thread, but I’ve been having the same problem trying to find this part or something I could use in its place.
I’ve ended up buying a cheap clothes brush, made of this, on eBay for £2.46 (free post), going to cut the material to fit the heads and superglue them on. 🤞
It’s crazy that these, or at least a sheet you could cut to size, don’t seem to be available anywhere.