It would really help if you put in your location. I can suggest some places in Florida. 1940s and 50s vintage knobs are rather easy to find here and 1920s and 1930s aren't very difficult to find. Other than that, Ebay, Craigslists, hamfests, and surplus and thrift stores and garage sales. I just picked up half of truck load of old MIL test equipment just to salvage the vintage GR and other type knobs and other bits from. I was back at the Source yesterday and they had some GR type knobs up to about 4 1/2 inches in diameter on some of the stuff in their scrap.
Look for groups like this one
http://clge.com/radiorelics/fawg.html and go their their swap meets. Tons of Vintage stuff there!
Oh and Skycraft in Orlando.
Post some pictures or links to pictures of what you're looking for.