Unless you're a 'purist', refurbishing to stock condition, you might check with your local radio/TV repair shops, and members of your local Amateur Radio club. These are always good sources for 'hard to find', and/or suitable substitute parts. There are several commercial vendors [such as Allied Electronics, Newark Electronics, Antique Radio Supply, etc] who stock a goodly choice of knobs. Another possibility {perhaps cheap, if you supply them with a sixpack of beer, after the job}, is your local dental lab technician [the folks who make a set of dentures, from the molds taken by a dentist. The technician could make a like knob, if you have a 'master' for him/her to make a mold from. They then pour acrylic compound into the mold/s, and produce an exact match. This can be a real chore, depending upon the intricacies of the 'master'. I've, on occasion when I'm desperate, made replacement knobs by casting a master, using the non-clear dual compound type of epoxies.