I use Addicted to black powder
http://www.addictedtoblackpowder.com/product/ as my primary source for real black , I only use real black in flint, cap and inline.
pyrodex is actually more corrosive than real black.
black stores for 3 life times and even if it got damp it can be dried it is actually made in a wet process in many cases.
the subs seem to have a short few year shelf life after the seal is broken if they are not stored in a very dry place before the seem to be degrade.
the guy to see about rocks aka flint is
https://www.trackofthewolf.com/List/Category.aspx/125 it is a small but probably most stocked shop around for anything muzzle loader or black powder and he ships quick is good to talk with over the phone if you have questions he is a one man operation I think but has a great selection of the otherwise hard to find stuff.
I also recommend a flint hammer the small tool will get you back to showering sparks in a few taps once you learn how to use it , you can really extend the life of a flint.
if you can go together on a case with friends that is the best deal for buying black then you can get it under 20 a pound shipped. you may think 2 pounds now will do you , but it stores virtually forever check local laws as to how much you may possess where I live 50# is legal in ILL 5# is max without a license. unless your a WI resident and driving your 50# back out of the state of ILL to take it home to WI.
I buy it for the youth muzzle loading program here in the county our last purchase was a 25 pound bag , it came as a cotton sack in a plastic bag I broke it down into cans we had saved. ( I also make the drive to go get it)
I have run FFFg in pan and as the charge you can use less of it as the main charge and especially just for target shooting , the big rock on a brown bess should shower plenty of sparks and it all lights at the same temp
remember your not trying to fuse the main charge your banking the pan charge so that it flashes through the empty flash channel to set off the main charge , I will put my flash channel pick in the flash hole during loading then remove it when charge the pan insuring I have a nice clear path.
I think historically many cut a feather to place in the flash channel and didn't charge the pan until nearly ready to fire a good plug in the flash channel kept the main charge dry even in weather when a tight fitting patch was used around the ball
if you fill the flash channel you fuse the charge and get hang fires.
properly timed a flint lock is as fast as a cap lock.
a pan charger is also very handy
https://www.trackofthewolf.com/Categories/PartDetail.aspx/83/1/FLASK-PP-4