I Assume that at least some old west cowboys reloaded their own cartridges, 38-40's, 44-40's, 45 Colts and even rifle cartridges.

I also assume they had to use the Black Powder they could obtain in towns that they traveled through.

How did they carry/store their Black Powder to keep it dry and usable until it was needed?

I have seen photos and read stories where they purchased the powder from bulk barrels and it was supplied in cloth or paper sacks, but that packaging would not have provided any type of protection from the elements, even in saddle bags.