You have a chance of finding a mimic whenever you loot something from an object, I.E barrels, brick piles, non locked chests, from inside a dungeon, normally they are kind of rare making them hard to farm.
However you can get them to consistently spawn if you head to the “fake” dungeon, it’s the big stone structure you should see past the closed gate you can see from the start of the town. Head inside the dungeon then start running around the tall blades of grass, you get items and cash from this as well as getting a really high chance to get into a fight with a mimic, they offer like 700 xp on floor 0 and more the further down you got.
One thing to note about this dungeon, there’s a bug that sometimes causes enemies on the floor to refuse to fight with you, and you cannot start a fight with them by interacting with them, this shouldn’t affect your mimic farming, but it stops you from progressing further into the dungeon as you cannot interact and fight the boss at the end of the floor. You can fix this by reloading the game, or by descending down a floor or 3 with the help of the plushie that occasionally spawns in around the starting area of a new floor.
Also, fun fact: If you interact with the barrel that starts the fight with the Barrel Master in town, there’s a small chance you can fight you can fight a Big Terracotta Mimic, I have no idea what happens if you win the fight, cause I got my ass whooped by her and couldn’t get her to spawn again.