I am always looking for original cars for sale, even though I am not always in the market. Team Camaro is limited on the classified section. The Yenko site has some, but again few. ebay and craigslist you will find advertised Z or big block cars but unverified , so scary.
A BB 1st gen or Z is to big of purchase to have guess work, IMO. I would contact Jerry Macneish and see if he perhaps knows of any for sale. He also checks out the Barret Jackson cars so it may be worth setting up a meeting with him at a BJ auction, where some known Z/BB cars are going to be auctioned off, and have him give you his opinion on which car(s) he likes and how correct they are. Any info you get from Jerry on a car he has seen first hand would protect you from a fraudulent BB or Z.
It may cost a bit more to go this route, but in the long term you will have a correct car the first time.