Timing belts are a fantastic way to transfer rotational motion (from a stepper motor) into linear motion (along a rail) and these GT2 belts are excellent for the task. They have a special profile with rounded teeth which reduces slippage. 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.
Timing Belt GT2 Profile - 2mm pitch - 6mm wide 1164mm long (11:37)