Black Powder is listed as an explosive by the ATF, shipping it requires a hazmat fee, sellers are required to have a special storage area away from everything else. These requirements have changed who sells it drastically, before the regs every gun shop had it, now very few feel it is worth the hassle. It is easily bought online and shipped to you, you do have to pay the hazmat fee.
Prices for black have just about doubled during covid, short supply and big demand have kept most of the online sellers sold out. They expect to restock by the end of this month, hopefully they will. The run on black has been the same as the run on modern ammo.
You can buy 1# or up to 50# on line, the hazmat fee (about $25)is the same for 1# or 50# so it is best to get some friends together and buy a bigger lot, we usually buy 25# at a whack. Yu can mix granule size in an order, it comes in 1F to 4F with the 4F being the flintlock priming powder. It never goes bad so you can store it for 100 years in the dry and it will go off like you bought it yesterday. I keep 15# to 20# on hand, some of mine is from the 70s and shoots just fine.
Black powder is corrosive but easily cleaned up with just plain water, with the barrel dried and oiled properly you will never have a bit of corrosion problems from using it.
Shoot it an leave your barrel uncleaned for a month or so and your barrel will turn into a sewer pipe. That is why there are so many older BP guns out their with sewer pipe barrels, during the first BP craze in the 70s many people just didn't know B/P took special cleaning procedures and treated their guns like they had their modern guns.