Back a ways ago, Mountain State Muzzleloading picked up what was left of the Browning Black Powder effort. I guess Mountain State itself has since become a casualty.
But, Cain's Outdoors has picked up such pieces as were left of MSM and is running with it. You might check with them to see if there is anything of that old inventory left.
Dixie Gunworks also might have something on hand that will work. Give them a call as well. Other than that, finding one will just be pure luck.