Thanks everyone for the great comments and advice. I should have mentioned that I built a percussion CVA Hawken 30 years ago and shot and hunted with it for a long time, so I'm not completely new black powder, but back then we just grabbed whatever pyrodex was at the sporting goods store and didn't think too much about it.
I hope you've got all that pyrodex mess out of your system. The flash point is too low to work in flintlock rifles, real blackpowder is what works, don't waste your time with the other stuff.
There are more than just a couple of vendors that will ship BP to your door so keep looking. Last time I bought any you had to get 25 pounds, now I think some places will ship just a few cans.
I just did a Google search for (Black rifle powder) and got so many hits I got tired of reading them. There are several types of black powder sold such as Goex, Swiss, etc. Some are faster than others such as Swiss, others are slower. But let me make a recommendation to a new shooter, start out with Goex black rifle powder. It's all I shoot and if it works for me it will work for you. Later on, once you've worked out all the bugs in your flintlock shooting you may want to try something else and that's fine but for now I'd keep things as simple as possible.