[magicalhippo]

They're not cheap, but not terribly expensive either. Adafruit has some[1] for $25, with 5V DC motors which is convenient. On DigiKey and Mouser there are some slightly cheaper[2] but with 10V motor.

[1]: https://www.adafruit.com/product/5466

[2]: https://eu.mouser.com/c/passive-components/potentiometers-tr...