You'll get more info here >> The sunn Discussion Page - Index as that is where everyone in Sunn world hangs out.
That said, the old Sunn grille cloth that doesn't look like Fender's is not available anymore. The stuff that looks like Fender's is available. You could try the Ampeg grille cloth that is available from Antique Electronics Supply, Antique Electronic Supply LLC . Aside from the Ampeg stuff, I can't think of anything that looks remotely close.