Both of my bulls were produced by Jason Nelson at Envy Reptiles. He has some amazing pits. John 1982 is a forum member and he has some awesome pits too.
Bull snakes in the US - Angel Argonza, Jason Nelson, Jonel Lopez, John Skager, Jennifer Joseph, Chris Kindred, Tim Spuckler, John Ginter all come to mind. Some are larger scale, some are more specialized, but I wouldn't hesitate to do business with any of them in the least.