There is a guy named Joe Wood on muzzleloadingforum.com that goes by the user name of flintsteel that sells them. I've read nothing but good reviews about his coning tool.
Here is a link to a thread he has that explains how to contact him to purchase one. FOR SALE - Muzzle Coning Tool | The Muzzleloading Forum
I used one of his tools on several .50 cal. guns and they worked as claimed. Did a great job. Used one to clean up a muzzle that someone had dinged up.