I tried to get some from Mcbrides once- they keep the legally allowed amount on hand- sometimes. My guy got there for his usuall visit (from waco) but theman with the key to the black powder safe wasnt there so, not soap.
Best is to order it from Powder inc, Clarksville Rod and gun or Grafs-if they still sell it. Some have minimum orders of 25 pounds but grafs and perhpas others would sell as little as four. Hazmat usually adds the cost of a can of powder to the bill.
Few stores have powder magazines and will get in four or five pounds at a time if they are willing to do that much. Generally, there will be a hoarder waiting at the door to buy it all up.