niter

[bp50cals]

Where do you find niter? I can't find it in mines or in a quarry.

[KingKickAss]

Strategic resources are hidden on the map until you unlock them, they have their own resource tile. If you have the tech that unlocks them and don't see any, you might have gotten unlucky with map RNG.
There's a little magnifying glass icon on the bottom left, use it to search up Niter and it will highlight every tile on the map that has it.

[Oaks]

Have you researched the tech that reveals it on the map?

There is a map search function, I recall down at the left hand side of the screen, you can type in a resource and it will show you where it is.

[grognardgary]

First it won't be in any structure you've already built unless it is under a city or a district. It could be in a mine provided you built one on a random hill that had no other resource associated with it. It will never be found in a quarry because those can only be built where a previously known resource exists. And there are no twofers in the game. other than that it will be found a random somewhere on the map quite possible somewhere at least mildly inconvenient.

[hyperion]

Use the map search feature. It seems to be the rarest thing on the map that you REALLY need.