The only one I've seen for a while is this William Greener 8 Bore ML Fowler from Puglisi Gun Emporium in Duluth MN. While this one may be of an earlier vintage than you have in mind, click on the hyper-link to see a very neat gun.

http://www.pugsguns.com/displayItem.do?id=1372

ps - I've been looking for a decent 18th century Flint or early 19th century converted to Percussion LONG single barrel fowler of US/English heritage as a display piece for many years. Anyone who has one for sale or knows of one please email me at: roger@nacz.com
Thanks

"I hate rude behavior in a man. I won't tolerate it" - Capt. Woodrow Call