Visit Thunderbird Cartridge Company, Inc. (TCCI) https://tbirdammo.com/product-category/powder/fuse/
In person, I mean. They don't list the black powder on their website, but they have it. I was there just last week and specifically asked about it. They are one of the only places you can get FFFF flashpan powder for flintlocks. Some of what they have is Pyrodex, and some is actual BP.
Don't try calling. They are waaaaayyyyy too busy right now. I found that out the hard way. They only way I was able to get from them what I needed and learn what was actually in inventory was to pay a visit. The entire time I was there, their phone was ringing off the hook. They were incredibly nice and patient with me despite the crush in orders they are trying to process these days.
They are in Laveen, on Southern, between 67th ave. and 75th ave., right where Southern banks and turns south. The address is on the website.