I would recommend silicon tubing from
www.mcmaster.com or
www.dangerden.com. Other than this make sure whatever tubing you buy is not placing any forces on the waterblock that could either leverage/rock your block off the cpu, or flex your motherboard.
I tried braided PVC in 1/2" from Home Depot, and it cost me a CPU! The vinyl thinwall product worked well, but sorta collapses on the intake side of the pump from the suction. I've had great luck with the 1/2" ID 1/8" wall silicon, and I hear the PVC tygon is very good as well and a little cheaper.
Hope this helps.