Since it seems hard to find real black powder, I'm considering making my own. Question: for the charcoal, I've read about how you're supposed to seal wood in a tin can or something, can you just use that store bought charcoal that looks like real chunks of wood? Or regular charcoal that isn't fast start?

I have looked into it but never tried it. From what I have researched Black Willow tree char coal is the best to use but pine will also work as the fuel. You can char it or any wood in a metal container sealed tight with a small hole on the top to allow gas out. Let it char for hours and when no more gas or flame is seen coming from the hole it should be done but don't open the can until it's totally cool to the touch. If it's not totally charred when you open it, put it back in the fire until it is. You can't over char it. Then you need salt peter and sulfur. All should be mixed in a ball mill separately and then when milled fine mixed, wetted and dried into pucks and grated over a screen to get the granulation you desire. It's a lot of work and time but I might try this some time in the near future. I'm betting the char coal made of real chunks of wood will work as long as it's milled to fine powder but not charcoal brickets like Kingsford for grilling.

buy BP at Graf and Son

check the archive, someone on here did learn to make their own and documented it.

thread

Best place to start is to look into the way pyrotechnicians do things. That's how I learned to make it.

In normal times I don't use enough to worry about making my own.

Here is a basic starter how-to. Realize of course that you'd have to corn and grain the powder to your desired size after making it.

I just looked and you can buy saltpeter on amazon.
The way things are going in our country I think the safest bet is a flint lock, home made powder, a t-shirt for patches and cast your own balls.

Try to find a feed store that sells potassium nitrate and sulfur. Way cheaper than Amazon. Charcoal is not hard to make. Get a metal gallon paint can from HD or Lowe.
You can use the hard wood  you use to cook with but it is slow burning. Willow, cotton wood and red cedar make good fast powder. You do need a way to  mill the holy trinity together
with out blowing up your house. I am lucky to have a place where I can mill 1.5 lbs at a time. Research and research some more.

Here is a kit for 10 lbs for Black powder for $52 + shipping I think.
From a fireworks co. Skyline
10 lb black powder kit

Skylighter - not Skyline. Great company and I've done business with them for decades. They do prefer people to have an FEL though - and they do cancel orders if you flag yourself in their system.

charcoal made from light wood vs heavy woods burns quicker.

I used pine 2×4 odds and ends the time I burned my eyebrows off.