Black powder can be difficult to find as few people use real black powder any more. You should have no problem finding Pyrodex in stores, but it works differently than black powder. You can use either black powder or Pyrodex for rocketry ejection charges, but black powder is easier to use and more reliable. Get black powder if you can. I got mine from a guy who has a muzzle loader. Keep in mind that there are storage requirements for black powder if you use it for anything other than muzzle loaders; and this includes people who have muzzle loaders.
If you have to use Pyrodex you need to make sure it is contained so that pressure builds up enough to eject your parachute. Just search this forum for how to use Pyrodex.