Everything in life is a tradeoff, tubing with thicker walls gives more kink resistance, but also more stiffness and liklihood of levering your waterblock.
To avoid kinks with thinner-wall tubing, be careful to ensure that your hose routing doesn't involve any tight-radius turns, or if you must make a sharp turn, use an elbow fitting there.
McMaster-Carr is a source of supply for just about anything you can imagine, including the kitchen sink, and every imaginable plumbing or tubing fitting under the sun. They list MANY different species of plastic and rubber tubing, PVC, silicon, you name it, with a variety of characteristics, wall thicknesses, hardness, etc. All sold by the foot or in large coils.
I can only speak for the Tygon PVC that I am using: McMaster part#: 5554K17 - which is 3/8 ID, 1/8 wall thickness, 5/8 OD, durometer Shore-A-55 (soft), seems quite kink-resistant due to the thick walls, but still reasonably soft and flexible and doesn't seem to lever the block any. One point in favor of the Tygon PVC, is that it appears to be as "stretchy" as silicone and can be made to fit onto barbs at least the next larger ID size if needed, (McMaster refers to stretchiness as "elongation".)
For your larger-bore 1/2-inch system, if you went with the clear Tygon PVC you'd need to choose between one of two available wall thicknesses, either 1/16th or 1/8th, I suspect the thinner-wall 1/16th would be a bit more likely to kink if you attempted to make too tight-radius turns with it, but it would also be the most flexible and least likely to twist your waterblock off the core.
McMaster part#: 5554K18 is Tygon PVC 1/2inch ID, 1/16th wall, 5/8th OD.
McMaster part#: 5554K19 is Tygon PVC 1/2inch ID, 1/8th wall, 3/4 OD, this is probably quite a bit stiffer though.
Dangerden appears to be selling silcone rubber tubing (at an outrageous price per foot though!)
McMaster lists 1/2inch ID silicone tubing in three available wall thicknesses:
McMaster part# 51135K41 - 1/2inch ID, 1/16 wall, 5/8 OD - this is thinner and more flexible, but needs more attention to routing to avoid kinks.
McMaster part# 51135K42 - 1/2inch ID, 3/32 wall, 11/16 OD.
McMaster part# 51135K43 - 1/2inc ID, 1/8 wall, 3/4 OD - this is the thickest, least likely to kink, but considerably stiffer as a result.
Check out McMaster's website and see what you think...