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)