Amazon.com: The perseids DIY Robot Smart Car Chassis Kit with Speed Encoder, 2 Wheels and and Battery Box for Arduino/Microbit/Raspberry Pi (2 Wheels) : Toys & Games
FK Engineering's Blog: Modeling and Control of Two-Wheels Self Balancing (TWSB) Robot (Updated 17 June 2023)
Mini Two-Wheels Self-Balancing Robot (Version 2 Prototype-1) - YouTube