Wilkinson's Caliper rebuild kit with Piston shows the spring, I'm not sure if they sell the part separately, but it might be worth a quick call to see if they'll sell just the spring ...
http://www.panterapartsusa.com...l.cgi?prod_id=01001E
I haven't tried moving it by hand, so I can't answer your second question ... I plan on replacing my cable soon too.
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