Good point Gary, although I think you would be looking at very late August if that.
So there is no magic search "the whole country" feature on craigslist. I will search the biggest metropolis areas around me ( SF LA & San DIego). If I was still in the midwest I would search chicago Cleveland/Cincinnati , Indianapolis. You get the idea. Another way to do it is type in to google " 1969 camaro 12 bolt sale craigslist" Or if you wanted to try to refine it "1969 Camaro 12 bolt BU rear end". Then any craigslist ad will populate with this info. I am sure there are many ways to skin a cat, this is just what has worked for me, (with some persistence).
Check out heart beat city, but I would visually inspect anything I got from them, after reading about prior purchases from different people.
One more thing did you get the protecto-plate or paperwork, that shows your car came with a BU coded rear end? Just curious