As a long time muzzle-loading firearms user, I figured to chime in here a bit.
First, several of my purchases have been on direct visit to Dixie Gun Works in Union City, TN, a couple hour drive from my house. They sell all grades of black powder across the counter, but only stock Goex brand I believe. They are a major dealer of replica black powder firearms, and every time I visit I browse for quite a while. They have an online catalog as well.
I use 1.5 Fg Swiss powder only, in a 1874 Sharps buffalo rifle (cartridge) replica. 2Fg I use in 50 or 45 caliber rifle applications. 3Fg is used in my Colt revolver replicas (percussion cap) in mainly 44 caliber.
I do have 4Fg, and that is used as only a pan priming source in my flintlock firearms. Very sparingly used at perhaps 3 grains only to prime a pan.
So I have a few decades experience in the use of black powder (as well as Pyrodex which I use in a modern in-line muzzle-loader application). I also use 209 shotgun primers rather than percussion caps for the in-line.
At any rate, a few years ago I made a bulk order via Powder, Inc. of Arkansas that was delivered by UPS I believe. HazMat applied of course, and I had to make a minimum order of 5 lbs. They were the only supplier that was reasonable on the Swiss powder, which again is reserved only for the Sharps replica (my "holy grail" rifle) cartridges. If memory serves, I had to send a PDF scanned file of my drivers license, and sign a waiver of some sort, which was all done via email. Once all that is done, your name and information stay on file there for future orders.
I'll close here with an amusing incident. I was firing my Kentucky long-rifle replica at the gun range one day (I'm a member of a private club here), and a group of AR-15 blasters were watching me with great interest. When I fired perhaps my 5th round of the day, they screamed out "ARE YOU OK!!". I looked over calmly after the shot and said, "Sure, I'm fine.". They were exclaiming "We saw fire shoot out the side of that thing, and thought you had burned your face clear off!" At this point they walked over, and I explained how a flintlock works with the pan and frizzen, plus the flint and hammer. I had rapt attention that day for a few minutes. But one statement got me...when I finished my explanation of how the flintlock system works, one of the younger members of the group asked, "Gosh, when did they come up with that?!" (as if it was some new innovation). I said, "Oh, about 400 years ago".