There are other powder sources other than making it, Estes powder and bottle rocket powder. One of them is buying black powder! If you are old enough (18 in most places) you can buy black powder at gun shops for black powder guns. There are probably places that you can order BP from, but I don't know what kind of shipping restrictions there are. You could always try to pay a homeless guy to buy it for you, but that might not work very wellÔø‡

Other fireworks can be sources of powder such as shells and cakes, but it isn't really worth it, you might as well just use them as they are. Firecrackers are not a very good powder source. Just try it once and you will see what I mean. It takes a long time for not much powder, but if that's all you've got, you might want to try it. I was exceptionally bored one day and took apart a 50-pack of firecrackers, getting 5.3 grams of powder. It worked, but not nearly as well as normal flash powder.

Firecracker powder: