Black powder has become difficult to come by in most areas, partially because most retailers don't want to deal with the licensing involved (or so I've been told) and partially because most people who shoot black powder guns prefer to use one of the multitude of black powder substitutes that are now available.
If you do find real black powder locally, be prepared to spend thirty bucks or more a pound for it; not that the substitutes are much if any cheaper.
You can buy black powder online and have it shipped to your home (subject to local restrictions), and its price is about half what most gun stores sell it for. But then you have to pay shipping and haz-mat fees, making this an unrealistic option unless you buy several pounds or more at one time; up to the federal maximum of 50 pounds.
One of the great things frequently mentioned about black powder though, is the possibility of making your own at home if you can't get it any other way. With that in mind, I decided to try my hand at making some workable black powder. My first effort, using the so-called "CIA method," was a failure. So I bought an inexpensive ball mill and tried again. This time, although I already had some potassium nitrate and sulfur, I decided to start from scratch and try making usable gunpowder from ingredients that were readily available from my local big box home supply store. Watch the video and see what happens.