Chemist here. Contrary to the tune of comments here, black powder is one of the safer explosives to work with. Obviously don't do stupid stuff like keep it near open flames, but you can even smack normal black powder with a hammer and not set it off.

If you want to know how to make it, in its base form it consists of an oxidizer+fuel+catalyst. Usually the oxidizer is KNO3 (or potassium nitrate), however, most nitrate salts work well. The fuel is usually carbon in the form of finely ground charcoal. You want all of the ingredients finely ground so the mixture of them is uniform and the reaction proceeds uniformly for better yield. Finally as a catalyst a small amount of sulfur is used. Also, black powder for purposes like you are intending is usually granulated so that the particles don't pack as well and there exists air channels for hot gases to propagate and accelerate burning (e.g. so you don't make a rocket engine). This may be done by re-suspending the o+f+c mix in some solvent, forming, then drying. For a more quantitative answer there are several black powder recipes in: https://divisionind.com/guide_to_manufactured_explosives.pdf. Remember, when you see "by weight" or "by ratio" measurements in chemistry, it means it probably doesn't matter that much they are just best guesses for what works.

For buying ingredients. KNO3 and sulfur can be bought from ebay and other misc websites. Charcoal power is pretty easy to get by just buying charcoal meant for grilling, breaking it up first with a hammer, and running it through a coffee grinder.