Your experience with the shooting of black powder vs. pyrodex, or any other substitute powder is much different than the experiences of other shooters. For different temperatures, relative humidities, and changes in weather, Black Powder is far better a performer than any of the subs, including Pyrodex, in both Flintlocks, and caplocks. That is without considering issues of cost, cleaning, corrosion, etc.
As for obtaining Black Powder, there are Goex Distributors for all the States, and a call to the 800 number for Goex, or the nearest distributor will get you directed to your closest retail source, if you insist on buying powder one can at a time. If you have been reading this forum for any length of time, you will have read countless thread indicating how to obtain BP from sources through shipping, and the prices you pay when you buy in bulk. It is wise to find friends to buy powder with, so you can order the powder at case prices. Its saves on the Haz Mat fee that is assessed against every shipment, regardless of the amount of powder shipped or ordered. The same suppliers often also supply percussion caps, and they can be included in your powder order for the same Haz Mat fee.
Go to the Links page on this forum( at the top of the index page, under "Member Resource) and find suppliers. McKeal has been posting a chart on current Powder Prices here for the last couple of months. There is a dealer in Lousiana called Jack's Powder Keg that currently sells Black Powder at the lowest price- case price amounts to about $10.20 per pound.
The only reason you can buy some of the substitute powders on the market from the Discount houses, is that the manufacturer is dumping the product in high volume sales to these stores just to move a product that informed MLing shooters don't want. Use it, if its all you have. And buy those bargains while they last.
This time of year, the prices are dropped so they can use their warehouse space to store something that will sell for the late spring and summer sales. They want to move that powder and get their money( and a smaller profit) out of it as fast as possible.