Cabelas and BassPro both sell powders just have to check to see what they have and it pays to ask at the counter they don't always have powder on the self. There is also a dealer out on Truman Road named Arms Mart 816.796.7514 or 796.
5800 they usually have powder if anybody in town does. If you happen to be down around Clinton the guys that used to own Kriesler Drug have a sporting goods store right next to where Kriesler drug was, this is over by the Walmart Center, they usually have about the best supply of powder and primers around and the best prices too!
RHB