the stuff comes in different brand names, e.g. Armaflex, Insulflex, etc. they all work the same. it's closed cell insulation foam, comes in tubular form with different inner diameter and wall thickness (typical thickness is 25 or 50mm) 25mm is adequate for most domestic applications. you can also buy them in sheets to wrap those large or odd shaped pieces.
make sure you select insulation with the right diameter otherwise you'll end up with an air gap between the tube and the insulation. this will cause condensation to form on the pipe which will fill the gap and eventually rot your insulation. also if you are installing this thing outside, UV rays will break down the foam eventually, so place a cover over them to protect them from sunlight.
if you only need a small piece, ask a ref shop if they could give you some off cuts.