http://www.mcmaster.com/
They make it easy to place an order on line. Its 1/8 I.D. hard Viton tubing (I don't recall seeing any soft Viton listed)

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body. '87 300D w/ Sportline suspension. No more '79 300TD w/ 617.952 engine at 367,750, it was my annual driver to Texas and I sure don't miss it poor A/C heater system!