Moyers Electronics would most likely have the part you need.
http://www.moyerelectronics.com/ However am hoping your RCA didn't do what Zenith did on some Chromacolor 2 and Early System 3 sets. They "cheaped out" on the Electronic tuners, and the knob was molded in a way that is was a permanent part of the tuner, if the knob broke off you were screwed as you had to replace the whole tuner assembly. Call Moyers and give them your model/chassis number. If a knob is available they will have it.