The Particle V1 knobs are designed for 1/4" round potentiometer shafts and have set screws for fastening.  They are a common knob available from many suppliers.  Here are some sources.

Small knobs, 16x14mm with set screw:

  • Davies 1140
  • SCI PN-8F
  • Alt part number: PN-38D-6.4
  • Eagle 450-2034-GRX (Mouser)

Large knob, 20x16mm with set screw:

  • Davies 1145 knob  set screw
  • SCI PN-8E
  • Alt part number: PN-38C-6.4
  • Eagle 450-2035-GRX (Mouser)