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From: At my Bar drinking and wrenching in Lafayette Colorado
You can buy a brand new reproduction manifold for about $200 from any of the Corvette parts vendors. And, as Jebby notes, you can buy used ones pretty cheap - there is nothing special about the stock cast iron manifold. A 5-minute eBay search should net you several options: there is a 1971 Vette driver's side exhaust manifold for sale there right now for $125. It took me 23 seconds to locate it, which is less time than you spent posting this thread and reading the answers....
I found two correct part numbers 350 OEM Manifolds on EBay that were less costly than the ones sold by Corvette Vendors. They were pretty rusty so I had them powder coated for $125. Just be careful if you buy ones that have the studs still on the manifolds, there is a trick to get them out without breaking them off. Use a Mapp Torch and then apply a wax candle once the manifold gets hot.
Thanks! I wasn't sure if there was any difference in quality of different repro brands. On Parts Geek, I saw several different brands (SKP, Dorman, TRQ, Davico, DIY Solutions, Replacement, Autopart Premium) but it seems like Dorman was the brand I saw on several other sites as well.
I will look into both OEM on Ebay and repro on the different Corvette sites.