Black powered is cheaper when you buy it in a five pound bag.
What I do is pour the powder from the bags in to empty cans of powder.
Be safe about it and keep your cans in a cool dry place and oil them
every now and then.

For those of you who like to hold on your black powder cans, a word
of caution. As the metal cans get older they start to rust up. If
the threads on the can rust there is a chance of a spark when you try
to force open the can. That spark and the black powder compressed in
a metal can will have an effect as a hand grenade. For those of you
who do not wish to feel the boom, I offer a simple suggestion. After
each use of your black power can take a little oil or petroleum jelly
to the threads on the can and also the lid.

Mark Wadsworth