Yeah, I know those locations by heart because of how often I see them. Three are hidden in the forest and one is hidden inside the middle trash bag inside the lab. For the codes in the forest, one is written on a stone plaque by the pond next to the entrance to where the ogre resides, another is written on a piece of paper inside a tent that's protected by two orcs patrolling outside (and it's also where you find the green? slime girl), while the third code is hidden inside a cave that's guarded at all times by a minotaur which you will have to defeat to gain access. An arrow made of rocks will point out where in the cave the code lies. I know you are aware of all this, but some people might not know about some of this, like the garbage bag and the paper. I only found the garbage bag one because I had just started the game and didn't know what was interactable and what wasn't, so I just interacted with almost everything.
Also, if you've made it this far into my rant, I'd like to ask if you can add some kind of way to grind for money in this game? The only way I've found to make quick money is by beating the champion in the contest hall, but it's just feels so slow and tedious, and it's especially annoying without the stuffing or gluttony ability (can't fill my character's stomach without them). I know I'm getting 1,200 gold per victory, but it's just feels so repetitive and boring, that it feels like it takes me half an hour just to get 5 grand.
I could probably just fight the custom monsters I can make, but that requires a crap ton of financial investment. 2,000 gold (Edit: 3,000 gold apparently) per capture net (which can only capture 5 monsters before breaking), capturing at least 10 different regular monsters to make each custom monster, it just doesn't feel worth it.