Amazon.com: DC 12V DIY Encoder Gear Motor 65mm Magnetic Gearbox with Motor Mount Bracket Engine Wheel Kit Micro Speed Reduction Motor Full Metal Gearbox for Smart Car Robot Model DIY(500RPM),Geared Motor :
Magnetic encoders : Encoders : Products : VS Sensorik GmbH
ODI - Magnetic Encoder Wheel | JIREH
Rotary encoders, Part 2: magnetic encoders
Position Sensors (Choosing the right sensor) - Zettlex Inductive Encoders
FAQ: How do magnetic encoders work?
Pololu - Magnetic Encoder Pair Kit for Micro Metal Gearmotors, 12 CPR, 2.7-18V