The better ones are homemade. There are all sorts of materials like milk cartons or truck inner tube rubber. I made mine out of a black, stiff but flexible enough plastic from one f the kids reading folders/file things.
There are rubber or leather in nes on the Internet, but they are quite small.
I used a bit of damp proof membrane (from a roll that I already had), attached to the mudguard with cable ties. Works a treat if I say so myself!