If you have exhausted most of the nationally known sources, try these guys... http://1967-81camarocarparts.com/
This is part of the Modern Automotive Group in Winston-Salem, NC. (I see you are from Georgia... Lived in Stone Mountain for three years, myself.) Modern Chevrolet has their own classic parts department. I know they used to stock some 250 stuff, and because of their inventory, it usually just pays to call and ask. --I'd call, not email. Lots of times, their guys will try and figure out where yoiu can find it even if they have discontinued it.
And good for you on keeping it original!