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After the leto's lab update , i found one in the Sunken Passage (dungeoun before Gorefist) by chance in adventure mode
I found one in that swampy area after the church and one of the rooms
Tangled grove on earth had one
The easiest way to get these is go through normal story , bellow some guy posted a very good guide of where to find all 4 on the map , use the stamina tonics from dog tooth , 1 piece of the drifter armor the wolf ring and whatever else you like , use letos amulet to make sure you are light weight and max all the stamina related perks and just literally run past everything and only stop to look in the locations the guy in the comments talked about . Getting the first 3 doing this takes about 45 minutes , so like 15 mins per simulacrum , getting the 4th 1 is kind of rng cause the scalding glade is split in 2 parts , 1 part after you defeat the Navun encounter and 1 part is locked behind a random dungeon with a mini boss, you can literally find it right after navun and get the 4th simulacrum on like 50 minutes , or have to search the entire scalding glade and go through 1 more mini boss + dungeon (though running through the dungeon is pretty easy , the boss is the real time waster) to get to the last part of the scalding glade and have it take like 65-70 minutes for the run. This goes without saying but if you plan to do Yaesha you also have to kill the Undying king every time so that you can skip nearly all of Corsus ( just make sure to accept at first so he opens up the portal and then kill him) The first 3 are always pretty constant and easy to get so either reset after getting the corsus artifact or just trust in your luck and go for the Yaesha 1 , doing this rounds down to like 1 simulacrum / 15-17 mins ish. I also used that world analyzer tool and kept re-rolling until i got the harrow on each run cause Claviger map is confusing as ***** and apparently it can also bug the simulacrum so F that.
Forgot to mention the earth 1 takes a lot longer than the Rhom/Corsus simulacrums cause of all the story encounters / forced mini bosses etc so it is not worth rerolling just for the earth 1 , in reality it takes like 22 minutes or so for the first +16 mins or so for the 2nd and corsus is really easy so it takes like 5-8 mins depending on if you kill Undying king or not or how good you are at the fight etc. it averages out to 15 minutes ish per , but the time is not distributed equally so it is worth going until at LEAST Corsus.
Are there any particular places on each world that will have one? I heard somewhere the only place on Corsus that can have one is the one where it's behind a locked door you need a key dipped in acid to enter. Or do they just appear at random either somewhere in the overworld or in whatever dungeon you get, and only some of the time? (I do get that they don't always appear and when they do there is only per world on each campaign or adventure)
From what I understand, in Adventure, there is a chance to get one or maybe even two per world, but you won't always get one and they could be anywhere.

In campaign, it seems they have fixed general areas they will always appear in, though the location in that area will be random.

On Earth it will be in the north dungeon accessed through the second overworld (after Root Mother), the dungeon with a boss at the end leading to the one-way gate.

On Rhom it will be in the world boss dungeon.

On Corsus it will be in the first overworld area.

On Yaesha it will be in the second overworld area. Most of the time it will be before the dungeon, sometimes it might be after, on the other side of the one-way door.
I actually got TWO on one Earth Adventure (one in overworld and one in Tangled Pass). I didn't know that was possible, I thought you'd only get one per world if any.
Dito. Got two - one on the "regular" world, and one in an mini-dungeon ! Was on earth, too.