For precise motion control, GT2 belts and pulleys offer excellent precision at a great price. Aluminum GT2 Timing Pulley 5mm - 36T is meant for use with GT2 6mm wide belts only - MXL belts will slip due to the different tooth profile.
Aluminum GT2 Timing Pulley 5mm - 36T has 36 teeth, and 5mm inner bore. Two set screws can be used to attach it firmly to any 5mm diameter shaft (like a stepper motor). Full aluminum construction means these are very light and very durable.
Adafruit Product Page: https://www.adafruit.com/products/1253