How could powder be in short supply? It’s charcoal, sulfur, and saltpeter. The first two are fairly abundant and the last easily manufactured. The colonists should have been drowning in the stuff.
Saltpeter is indeed the limiting factor. It is easily manufactured in small quantities. It is producing it in large quantities that is difficult.
All over Europe, governments attempted to organize every available resource to make saltpeter. It was still expensive. Most of the gunpowder used by the Americans in the revolutionary war came from France.
In 1800, a Frenchman, Dupont, immigrated to the United States. He had studied under Lavoisier, ‘the famous French chemist, who industrialized saltpeter production in France.
Dupont revolutionized gunpowder production in the United States, but it was after the Revolutionary war.
After the war of 1812, saltpeter in caves in the U.S. were found and utilized.
Finally, large quantities were discovered in caves in India, and on dry islands where bird guano had been deposited for thousands of years, off the coast of Chile.
They supplied much of the world for decades.
http://firearmshistory.blogspot.com/2016/03/the-history-of-saltpeter-v.html
Potassium hydroxide plus nitric acid yields potassium nitrate — saltpeter. Potassium hydroxide can be made from wood ash. Nitric acid is a little trickier but could be synthesized from ammonia.
But that is looking at history through a modern lens ...
The Confederate powder manufacturers collected the contents of chamber pots to manufacture saltpeter with.
Mr. Dupont manufactured most of the gun powder used by the Union army and navy during the Civil War. His product always worked. This was not necessarily the case the stuff General Gorgas was making for the Confederacy at Augusta GA.