You don't live too far away; less than 8 hours. Get your waders and head down there and find two of them so I can have one too! I would prefer mine had the sabot if you can manage that.
But seriously, does anyone know what the target the few that were found was fired at and what CS battery they were fired from? Looking in the book on Island #10, I see that it lists 10 of the 24 guns on the mainland as being "32-pounder rifles" which I'd assume are rifled and banded 32-pounder smoothbores especially since it lists "32-pounders" separately. It also lists a "32-pounder rifle" on a floating battery. There were a good deal of various size cannons on Island #10 proper but none seem to be the right caliber unless the "128-pounder rifle" is a 6.4 diameter piece. This is a long winded way of saying that there are a number of batteries that could have fired them. If I knew where to hunt I'd go find you one.