Pre-war money is an item that can be found quite often in the ruins of populated areas such as towns and cities in Fallout 76. That said, although the money no longer carries value, players can still use it to make cloth, which in turn can be used to make ammo if the player uses it to make gunpowder.
So if this is what you are after, you find pre-war money very commonly in safes, cash registers, on dead bodies in Flatwoods, and during both the cargo bot and back of the beat events. In the latter, there will be a bank building that you can loot a few stacks of the old money.
Once you got pre-war money, you can either sell it or scrap it to turn it into cloth. We highly recommend going with the scraping route, as pre-war money doesn't sell for much, and you can get more use out of it by scraping it.
That being said, you can scrap pre-war money at any crafting station in the game, and you can find said crafting stations in various locations, including your camp and stations across Fallout 76.
When you find a station, just walk up to it and hit the button that shows up for scrap. After this, hit scrap all junk items, and you will be good to go
For more in Fallout 76, check out What is the Fallout 76 Expeditions: The Pit release date? and Where to find Feral Ghouls | Best Fallout 76 Feral Ghoul locations