Where I am, pantry moths are the problem. One of the few genuinely useful ideas I've ever gotten off NextDoor is this: Dr. Killigan's Premium Pantry Moth Traps with Pheromones.
The pheromones attract the moths, and the glue makes them stick to it and die. You can put it out of site in a cabinet, and after a few weeks it's covered with dead moths.
The other option is to put things individually into well-sealed tubs or jars, to isolate any moths that might be there (they will drill a hole through thin plastic, like ziplock)
Honestly the trap method makes me a bit squeamish; if you’re continuously catching new moths, that means there're still eggs and larvae in at least one of your bags of food. The idea of eating that is a bit disturbing is to me.
You still have to find the source as you describe. However, if one of the moths flys out and away from the cupboard, they still might lay eggs some where else. You want the new hatchlings to go straight to the trap and get stuck.
We have had some moderate success with the traps, plus storing big bags of rice and flour in picnic coolers and everything else in airtight containers, but they have to be really airtight -- those little buggers can get under the lid of a mason jar or the tops of a canister set. What happens now is like a lesson in public health and contagion: one container will occasionally get a mess of webs and moths, but the scope is limited.