Finding saltpeter is actually quite easy. Any fireworks supplier like Skylighter has it. Also many pharmacists have it in 1/4 lb jars. The fireworks grade product is probably higher quality and certainly less expensive. Also, any place that sells sausage and ham making supplies has it. A lot of fireworks hobbyists make their own powders, including the black powder booster charge. I don't know the exact legalities, but apparently it's legal to make limited quantities.