Any container with a recycle code of 2 is good. Find a friend who is into Protein shakes. Theose containers are nearly identical to gunpowder bottles.
I stopped using wood entirely. Go through your wardrobe / towels / bedsheets for anything worn out that is 100% cotton. One pair of jeans makes enough charcoal for about 20 Lb of powder, and its even better than toilet paper. And basically free as you can always convince your wife you need new sheets & towels.
I still sort my powder into seven different grades because thats just part of the fun. But then when I run out of the right size I just use the closest stuff I still have some of. Even if it gets to where I am firing shotguns loaded with 1C. Hey, it worked during the French Revolution (they used canon powder in everything)