i go to richs outdoor world in summerville,pa for powder call ahead and they will reserve you some.Here in Pa I go to Dixons in Kutztown. It' a small muzzleloading shop which usually stocks not only Goex, but Swiss, and sometimes Elephant powders in all granulations.
They have a website, that may answer any questions.
As for blackpowder substitutes, 777 is decent.