Single shots are where you find them. I'd watch the bigger auctions, Rock Island, Amoskeage, Little John's, etc. Then look over Guns International, Gumbroker, and your local area gun shows. The only one (3rd model) I have I found at a local area gun auction, (500 gun auction) that a local auctioneer has about twice a year. Become friendly with all the gun shops, and let it be known you're looking for one, and to let you know if one shows up. Be prepared to spend 4 figures, and for a 4th (straight line) be prepared for mid 4 figures.
Just noticed your SE Iowa, You may not be too far from the gun auction I frequent, in Hamilton, IL, just about 20+ miles from Keokuk, IA