The insulation you want is closed cell elastomeric tube insulation. The closed-cell structure creates exceptional thermal properties (k-value of 0.245 at 75F and wvt of .03 perm-in). It's widely used in the HVAC/R industry. It's available in a wide range of diameters and wall thickness. It will not be available at the big box building stores. A HVAC supply house will have in the common sizes for A/C piping sizes. Depending on the supply house they may have to order the sizes you need. One manufacturer is K-Flex. See: http://www.kflexusa.com/
Do an engineering heat loss calculation to determine what wall thickness you need. OR, buy the thickest wall you can afford. Make sure whatever you choose is closed cell. Get the manufacturer's data sheet, don't trust the salesman B.S.