Duda Diesel is where I get both my pelleted Potassium Nitrate and flour Sulfur. Seems to work just fine and fairly fast delivery.
I use an electric coffee grinder to grind down the pellets until they pass my #40 screen. And like I had posted just recently, I had much better results with very little clumping of the combined ingredients in the mill by toasting the Potassium Nitrate in cookie pans in the oven under low heat for an hour or two before mixing in the final mill. The drier the ingredients are, the better for the final mill.