Re: Where can I get loose black powder?
Revolvers don't use patches. The ball is sized to fit snugly in the cylinder and then is squeezed into the barrel by the forcing cone. Single shot guns usually use a patch to make a tight fit between the (slightly) undersized ball and the barrel. Perhaps your derringer is intended to be shot with exact fit balls, but I would be surprised.
Get a copy of Lyman's black powder manual. This is, in my opinion, the best of the books out there to introduce you to the art of "shooting dirty." Most 50 cal barrels are designed to use 0.455 to 0.458 balls, with a 0.015 to 0.010 patch. The patch is lubed with a light grease (Wonderlube, Natural lube, lard, Crisco, etc.) worked into the weave.
I would contact the manufacturer and see if you can get a copy of the owners manual that came with the pistol. At the least, they should provide you with loading and care intructions.