SM465 shift Knob

[tucsonjwt]

I tried to unscrew the shift knob on my SM465 manual trans shifter in my 83 C20 and the knob won't come off, and now it has a long crack in it. There is a nut which appears to be pressed into the knob, which then threads onto the end of the shifter. 2 questions:
1) How should the knob be removed - turned counterclockwise like a normal knob, or unscrewed by the nut partially visible on the bottom of the knob.
2) Where can I get a stock knob?
I am getting ready to replace the carpet, which is why I am messing with this at all. Thanks