Black Powder for Rockets

I am part of this rocketry association and we really wanted to expand our rockets by experimenting with black powder. Though we will be launching our rocket in a supervised and legal zone, I am having some trouble finding the black powder. Does anyone have any clue or idea where I can get such materials?

Edit: I live in Ontario, Canada so any company or supplier that ships around there would work well for me. I’m willing to drive and pick it up if it’s in Ontario.