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 1162mm long (581 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.