Rick Armstrong, in Gore, Va., known on eBay as revwarcannonballs (and formerly known as The Gallant Pelham), who is a charter member of "The Biggest eBay Crooks Ever" had a genuine Fort Sumter cannonball, with written provenance to boot, for sale several years ago.
If you let him know that's what you're looking for, I'm sure he can rustle up another.