The belt itself can be found on Ebay, the pulleys are generally custom made by Harry or myself. If you can fit them, HTD3 6mm wide timing belts will be a lot more consistent and infinitely tougher. I would strongly suggest having a smooth pulley (again, can be made) on the weapon end for slippage under heavy load. Protects the system.

edit: saying that the 4mm round belts are perfectly viable (use them on my bot) if they are well shielded, it's just that they pop off quite easily if caught by another machine.