You can buy a tank from RockAuto. They were the cheapest place I found. You want a "Spectra" tank and that's what RockAuto sells. For brake and fuel lines I'd look at Fine Lines. Inline Tube also sells lines but I've heard of complaints with fitment.
Brake parts are still available from a regular parts store. I got a reman master cylinder, brake hoses, pads/shoes, rotors, a spring kit and wheel cylinders from my local parts stores. 71 uses a different brake rotor then the later years so you might not find them at a parts store. You can change to the new style spindle and run the cheaper rotors or......contact "The Ram Man" for rotors with the original size bearing.(they are expensive)