Timing belts are a fantastic way to transfer rotational motion (from a stepper motor) into linear motion (along a rail) and GT2 Timing Belt is excellent for the task. It has a special profile with rounded teeth which reduces backlash. GT2 Timing Belt is often used for precision 3D printers and CNC machines.
This belt is 1164mm long (582 teeth on a 2mm tooth pitch) and is 6mm wide. It comes in a loop but you can of course cut it down if you need a shorter length.
Adafruit Product Page: https://www.adafruit.com/products/1184