I get it and all other hose, clamps, and fittings I need from a local industrial hose supply place.
Mako Hose & Rubber Co.
They call it silicone heater hose for water cooled motors.
Google it and there's alot of places that sell it online in blue, black, or red reinforced silicone too.
It is more pliable and flexible. It's easier to push on to and pull off hose fittings. It also doesn't get hard, crack, and dry rot like plain black rubber heater hose eventually does and is more heat resistant.
edit: fwiw, here's the Mako page with silicone hoses
http://makohose.com/?page_id=360