Dropped by
spend the early ones carefully
Every locked chest eats a key for good, and the first pass through the Dungeon rarely turns up enough of them for everything you find. Note the ones you had to leave shut and come back, because the supply keeps arriving from the same enemies long after the loot inside has stopped being relevant.
the slime is the reliable source
Ordinary Dungeon enemies part with keys rarely enough that a normal clear leaves you short. The slimes that wander the halls always carry one, so a farm built around them is what turns keys from a bottleneck into something you have a stack of. Pots are worth smashing on the way past, but they are not a plan.
hold onto the surplus
You can end up holding one before you are allowed to explore properly, and it simply waits in your inventory until you have the run of the place. Later they open the fished lock boxes and the shaking chests that let a certain slime out, so do not sell the extras once the chests are cleared.