Mayhems tubing is mega cheap on overclockers, something like £1.69 per metre for 16/19mm. Seeing as it's usually Mayhems coolants that can be funny about tubing material, it makes sense to use their tubing if you plan to use their coolant. EK and most of the other big watercooling component suppliers all have their own range of tubing, so match the tubing and coolant companies and you'll be laughing.