What is the quickest/easiest way to obtain gunpowder?

Aside from using server commands, there are only four ways to get gunpowder:

• Creepers
• Ghasts
• Dungeon Chests
• Witches

Of those four, creepers would be the easiest way to reliably (and safely) get gunpowder. Your best bet would be to make a mob farm (also called mob traps or mob spawners). The general idea is to set up a dark room for mobs to spawn in, and have them funneled into an area where they are either easy to kill or die due to the environment or traps.

This question covers how to make mob traps, which I also linked above.

The easiest way is to build a mob spawner and just afk overnight. Otherwise yes, I wish you happy creeper hunting.

What I saw from a video is that the best way to get string/arrows/bones/gunpowder is to make a mob trap. To make a mob trap you need to have a sealed, very dark and very high up room with no torches and no doors and you also have to have water flowing into a hole in the middle, the hole has to be deep, at the bottom you have to have a bit built out that's got lava and signs to hold the lava, once you've done that then you build a bit out and put water down so that when a mob/mobs spawn they fall down the hole and the water pushes the mob/mobs towards the lava so the mob/mobs die and the item(s) get pushed towards a point ready for you to pick them up.