In 2013 there were sources for 'reproduction 69 Z28 radiator cores', but like every other 'reproduction' part, there are always differences from original. At that time, the difference was in the top/bottom rails (repros did not have the square holes as did the originals). I'm trying to find my notes on the person I spoke with and the details, but no luck so far... I'll continue looking!).. In the meantime, I'm pretty certain that I began with 'Joe' at Tanks and Tabs, 801 Fesslers Lane, Nashville, 800-874-2559 (recommended by Steve), and went from there. The other thread mentioned that Joe was getting his cores from Northern (northern radiator?) and I'm fairly certain that I got a contact for Northern from Joe, and spoke directly to the core manufacturer... a lady from MS originally, but at that time the only shop they had that would install customer top/bottom rails to their cores was in Texas... so that was where my rails went for installation onto the repro core...
Note: re the comment on 'oe style fins'... I assume he means the fin pitch, which on my original was between 12-14 fins/inch... and is matched on the repro core.