http://www.thunder-ridge-muzzleloading.com/buffalo.htm#balls

Thunder Ridge has roundball on sale. The .575 are the cheapest I have seen. They also carry minnies of all weights for that rifle. You might want to buy out the roundball if you like as I see they have a limited supply. I was thinking of doing that myself, but I just bought four hundred of them and a mold to make my own.